With his fourteen preparation of photosensitive material probably made in the first week of February 1839, John Herschel introduced gallic acid into photographic science. His hopes of the gallate of silver were communicated to W. H. F. Talbot in a letter of 28 February and indirectly through J. B. Reade towards the end of March 1839. Gallic acid was mentioned at a reading of Herschels paper on photography at the Royal Society on 14 March 1839, and the somewhat problematic lightsensitive combination of gallic acid with silver nitrate published by Herschel early in 1840. Unpublished photographic use of gallic acid by J. B. Reade in late March 1839, and brief published references to the chemical by A. Smee and G. P. A. Petzholdt during 1839 had little significance. Talbot purchase gallic acid on 30 March 1839, tested it six days later, and in 1841 patented the calotype technique in which gallic acid acted as a physical developer. Chemical ideas relevant in 1839 to the predeveloper use of gallic acid are discussed.
Evaluation of W. H. F. Talbots contribution to the invention of photography has often been a source of discussion amongst historians and no doubt will continue so to be. Without entering into the reasons and assessments for the view for many pages would be required this author believes that Talbots part in the birth of photography in 1839 has generally been overrated in England. Talbots more significant contribution lies after 1839 with the technique of using gallic acid as a developer in the calotype process patented by him in 1841. The origins of his first experiments with gallic acid are therefore of particular interest in the history of early discoveries in photography.
Although the subject has been discussed in two earlier articles on Gallic acid and Talbots calotype patent , , and as the field of study has been notoriously open to speculation but lacking supporting facts, the intention here is to provide additional data from more recent research on the influence on Talbot in 1839 regarding gallic acid.
In midJanuary 1839 reports appeared in the newspapers in Paris that Louis Daguerre had discovered a method of obtaining permanent pictures from the images of light displayed in the camera obscura. In England Talbot heard of the announcement made in Paris Academy of Sciences of Daguerres discovery and as soon as possible displayed at the Royal Institution in London some photogenic drawings he had obtained with the printout process that he had been experimenting with in the previous few years. He quickly prepared a paper on the subject for immediate publication in weekly journals and for submission to the Royal Society. His paper was read at the Royal Society meeting of 31 January 1839, although the council of the Society were unhappy that Talbot made only generalised remarks without any technical description and they were no happier that he had already sent the paper for publication in the general press.  Talbot was persuaded to submit a further paper giving technical details of the photogenic drawing process and it was read at the Royal Society on 21 February 1839 and published immediately in the Athenaeum on 23 February.
Sir John Herschel on hearing that Daguerre had obtained pictures using light immediately started experiments at the end of January 1839. With admirable ease he had in a few days solved the greatest problem of preservation and fixing by using hypo. Talbot and Herschel corresponded about the new discoveries. Talbot was always very restrained in giving any technical details of his own work for example, he withheld from telling Herschel of his own inferior method of fixation/stabilization until his own paper was read at the Royal Society on 21 February. In contrast Herschel was very open about his work. In a long detailed letter to Talbot on 28 February 1839 Herschel wrote: 
|I had been trying various modes of rendering [silver] nitrated paper more sensitive till I read your most curious count of your process, which opens up quite a new view of the subject, and is altogether one of the most singular things I ever saw. You must have hunted down the caprices of these combinations with great perseverance patience. When I read it I gave up further trials, your processes being very simple and complete I had most hopes of the Gallate of Silver [i.e. Silver Nitrate + Gallic Acid], which is affected by light very differently from its other salts...|
In spite of the immediate welcome given by Herschel and others to Talbots photogenic drawing technique, it was soon realized that it was not, in fact, a particularly satisfactory method. Paper treated with a weak solution of sodium chloride was, after drying, given a wash with silver nitrate. A partial silver chloride was thus produced that was more sensitive to light than simple silver nitrate. [N.B. Silver Nitrate is water soluble; Silver Chloride is not water soluble] Talbot also recommended further consecutive treatment with the same solutions before exposure. If, after exposure, the prepared paper was treated with the strongest saturated solution of sodium chloride a complete silver chloride was produced that was stable to light. It was therefore a very capricious method and the eager early photographers were soon disenchanted.
Herschels experimentation in photographic science was from the beginning thorough and rewarding. He recorded details of Batch preparation of his experimental photosensitive materials in a Photographic Memoranda book  from 1839 to 1845 and in 1859. He prepared 670 sensitive papers in 1839, but unfortunately the first one to be dated was not until No. 421 on 16 March 1839. Even so the Photographic Memoranda book does show that Herschels first experiment with gallic acid was amongst the very earliest photographic work carried out by him. It was his fourteenth preparation of photosensitive material: being recorded on the first page of this Photographic Memoranda book as
|14. (Nitr Sil + Gallic Acid) 1 wash. (anomalous) N.B.|
Probably this preparation on nitrate of silver and gallic acid was done in the first week of February 1839. In the passage quoted above, from the letter written on 28 February to Talbot, Herschel does not speak as if he had used the gallate of silver only recently.
Talbot might easily have missed paying much attention to Herschels brief words about gallate of silver in a letter packed with interesting details. But he did not do so, especially when the same chemical was mentioned to him again just one month later; as we can learn from some correspondence Talbot had with J. W. Lubbock. Sir John W. Lubbock (180365), Treasurer and VicePresident and of considerable influence in the Royal Society, wrote on 27 March 1839 to discuss with Talbot the mode of action of hypo as a fixer. Talbot replied by return of post on Thursday, 28 March 1839. 
Q. Ann St. Thursday Evg.
I am very much obliged to you for the Comptes Rendus [the French scientific journal devoted to reporting the proceedings of the Paris Academy of Sciences] which you sent today, as well as the preceding on. Biot seems to take a great interest in the sensitive paper...
I recommend to you to try the sensibility of paper washed first with nitrate of silver, & then with gallic acid the latter to be pure. I have not tried it yet, but it is [sic] has been mentioned to me both by Herschel & another experimenter, so that I think it must be among the best recipes yet found out.
Herschel told me that the hyposulphite washed out the chloride of silver; but I have certainly found that the hypo preserves the [photogenic] drawing if merely spread over it, and not afterwards washed out...
Yours very truly,
[P.S.] We leave town next Tuesday [2 April 1839] for Lacock Abbey. When in the country I hope to find more leisure for experimentation on this subject
The use of gallic acid with silver nitrate had been mentioned to Talbot by 28 March 1839 by Herschel and another experimenter. Many persons carried out experiments with the new art of photography in the first six months of 1839. By Easter 1839 photographic paper was on sale in London shops. J. T. Cooper, junior, the chemist at the Polytechnic in Regent Street, was preparing paper on a commercial scale and had advertised each week in The Athenaeum since 16 March that the paper was on sale at chemist shops in Regent Street, Charing Cross and the Strand. On 2 April Thomas Dawson, a printseller of Bedford Street, Covent Garden, was also advertising packets, at 6 shillings each, of eight sheets of photogenic paper, as prepared by H. F. Talbot, Esq., F.R.S. According to the advertisement in The Times, the prepared paper was obtainable through all stationers: be particular to ask for Dawsons Photogenic Drawing Paper, advised Dawson. it being made from the original receipt and therefore may be depended upon. Dr. Diamond, many years later a secretary of the Photographic Society of London, was one who in April 1839 brought some commercial paper to begin their photography.  But many were more adventuresome to do it themselves: in London, amongst the names we know, simply because they published articles after experimenting, were C. T. Downing, Alfred Smee, the artists J. T. Wilmore and William Havell, who all published in the Literary Gazette, and Golding Bird (in the Magazine of Natural History). By 8 June 1839 the weekly Athenaeum was complaining: Hardly a day passes that we do not receive letters respecting imagined discovery, or improvement, in the art of photogenic drawing, but the suggestions are generally far too crude to be worthy of publication.
Who amongst the known (or unknown) experimenters was the another experimenter who had mentioned gallic acid to Talbot late in March? Some time before the middle of May 1839 Alfred Smee,  an ophthalmologist in the City of London who took an active interest in many branches of science, had tried gallic acid amongst many substances unsuccessfully tested for fixing (not sensitizing) photographic paper: in view of the use to which he put gallic acid, it is unlikely to have been Smee who spoke to Talbot just before Easter 1839. Indeed, Talbots contact was, indirectly, the Rev. Joseph Bancroft Reade, F.R.S. (180170). 
Reade was one of many who began their first trial of the infant photography after the public announcements in January 1839 by Daguerre, and soon after by Talbot. Reade, like others, found Talbots photogenic drawings technique to be less practical than the extent to which expectation had been aroused. Indeed, experimentation by all was necessary in 1839.
In the late 1830s Reade was very interested in microscopy and related optical sciences, but also had an abiding interest in chemistry. He read a wide range of scientific periodicals and occasionally had short papers published. In 1836 and 1838 he communicated two papers to the Royal Society. The referees  who were asked to review these papers did not think they were of outstanding merit, but they were read at meetings and slightly shorter versions were published in the Proceedings of the Royal Society. He was elected a Fellow of the Royal Society early in 1838 and his attendance at their meetings was at its peak in 1839 and the early 1840s. Reade was obviously at the two meetings of the Society when Talbots papers were read early in February 1839 and saw Talbots photogenic drawings on view. On 28 February 1839  he wrote to the secretary of the Royal Society a short account of his first trials and modifications of Talbots photogenic drawing technique, but it was slight work which quite reasonably was not read at the meetings of published by the Society. No information exists concerning the possible refereeing of his letter of 28 February but with Herschel as the obvious man on hand as a referee it is conceivable that it could easily have led to some contact between Reade and Herschel. But there is little doubt that Reade [11 bis] was at the meetings of the Royal Society on 14 March 1839 when Sir John Herschels fine paper on photography was read. Herschels paper was of particular importance and influence because hypo as a fixer was suggested in it. But he also mentioned that he had found some silver salts, other than the hardlysensitive chloride, were especially responsive to light; they were the carbonate, nitrate and acetate and in which the acid being more volatile ... imparts greater sensibility to the paper on which they are applied.  When his paper was published only nine days later in The Athenaeum of 23 March 1839 the particular sensitivityincreasing volatile acids that he had in mind were not specified. It is possible to compare the published version of his paper with the contents actually read at the meeting. The manuscript version as read is certainly more wordy (four times as long), but it is hardly more informative; indeed Herschels paper as published is not only outstanding for its contents, but also for its model editing. However, there was one, or rather two, small pieces of additional information given at the meeting. Herschel briefly mentioned at the meeting that he had found ferrocyanate not to be suitable as a fixer, and the important item that did not appear in the printed version was exactly in regard to our own interest in the sensitivityincreasing volatile acids, where the verbal aside was added being of animal or vegetable origin (especially the Gallate and Urate). 
J. B. Reade continued experiments on photography during the rest of March and it may have been as late as Tuesday, 26 March 1839, that he first used (or perhaps was really first successful with) a wash of gallic acid onto paper treated immediately before with silver nitrate. Fifteen years later, when he gave evidence at the calotype patent lawsuit of Talbot v. Laroche at the end of 1854, Reade stated he had spoken in 1839 to his acquaintance Andrew Ross about the use of gallic acid. Ross, the optical instrument maker whose shop in 1839 was in Regent Street, London, had soon after mentioned the fact to Fox Talbot. The Rev. J. B. Reade, close to his thirtyeighth birthday in 1839, was then living at Peckham, 5 miles out of London, with his wife, Charlotte, and two young daughters (neither of whom survived into adulthood). At the very end of March he suddenly decided to visit his fathers home in Park Place, Leeds, during the Easter holiday. He was in Leeds on Friday, 29 March, and travelled back by railway (via Manchester and Birmingham) on Wednesday, 3 April 1839.  His conversation with Ross about gallic acid must (from a comparison with the situation of W. H. F. Talbot) have surely been on 28 March, the day he started out from Peckham and London to go to Leeds.
In the evening of Thursday, 28 March 1839, Talbot wrote to Lubbock the letter quoted in full earlier in this article saying with regard to gallic acid: I have not tried it yet, but it has been mentioned to me both by Herschel and another experimenter, so I think it must be among the best recipes yet found. It is tempting to think, therefore, that Reade and Talbot had both spoken to Ross on the same day of 28 March. Unfortunately the events do not seem to have been quite so straightforward. Talbot undoubtedly visited Rosss shop in Regent Street on the 20 and 30 March 1839 , see Fig. 1:
Fig 1. Andrew Ross & Co., Statement of Account for 1839.
Shows Talbot purchased items on 20 and 30 March 1839.
(By courtesy of the Talbot Museum, Lacock)
It therefore seems most likely that it was on the 30 March, when Talbot purchased a solar microscope/camera and an objective lens, that Ross spoke to him about Reades remarks concerning gallic acid. If so, on 30 March 1839 Talbot must have also gone up Regent Street into Oxford Street, for on this same day he purchased 1 dram of gallic acid in a bottle, at a cost of 1 shilling and 6 pence, from Alexander Gardens chemist shop at 372 Oxford Street. Talbot travelled down to Lacock, in Wiltshire, on 2 April. Within three days he there carried out a test with the gallic acid recently purchased and entered into his Chemical Notebook: 
|April 5 Lacock Dilute gallic acid & dilute nit silver mixed turn dark in daylight (I believe Mr. Reade discovered this).|
Obviously a difficulty stands in the way of full acceptance of this timing just proposed for Talbots conversation with Ross because Talbots letter to Lubbock shows that he was already on the evening of 28 March aware of another experimenter. Speculation that it was not Reade is unrewarding, although it does seem ultimately difficult with regard to the use of gallic acid in early photography to avoid considerable speculation.
Talbots undoubted visit to Ross on 20 March is extremely unlikely to resolve the problem because this date appears to be too early for Reades first experiments with gallic acid. One explanation which is more reasonable, unless he learnt of Reade in an entirely separate earlier way, is that Talbot did indeed visit Ross on 28 March and learn about Reades use of gallic acid in time to mention it to Lubbock that evening, but that his visit to the shop on this occasion was not recorded because he did not make a purchase.
The sequence of events leading to the introduction of gallic acid to W. H. F. Talbot in March 1839 resulting in his first test of the effect of this chemical on silver nitrate on 5 April 1839 has now been described and the immediate terms of reference of this article completed. But some additional information regarding gallic acid in photographic studies later in 1839 and in early 1840 can provide a little more background to the period prior to Talbots use of this substance as a developer of the latent image in September 1840
A few days after returning from Leeds to Peckham, Reade wrote on 9 April 1839 to E. W. Brayley (180270), librarian and lecturer at the London Institution in Finsbury Circus, City of London.. Brayley was also London Editor of the monthly scientific journal, the Philosophical Magazine and as Reade had had other work published in the journal he may have hoped to have had accepted for publication a communication on this experiments in photography. However, whatever the original form of contact between Reade and Brayley, interest was obviously expressed because Brayley was to give a lecture at the London Institution on Wednesday 10 April 1939. Years later, at the time of the Talbot v. Laroche lawsuit in 1854, Brayley made a statement that
In my lectures on Photogenic Drawing delivered respectively at the London Institution on 10 April and at Walthamstow on 2 May 1839 I ... formed and exhibited Gallate of Silver by adding tincture of Galls to a solution of nitrate of silver. I also explained the action of light upon the gallate so formed and its consequent use in photography as devised by the Rev. J. B. Reade and recommended by him in his letter to me of 9 April 1839. 
Brayleys lecture was never published. However, eight years later Sir David Brewster (who, incidentally, amongst his many activities was the chief Conductor of the Philosophical Magazine, and provides a link between Reade, Brayley, and a personal acquaintance with Talbot  ) published an article on Photography in the North British Review of August 1847 and quoted a long extract from Reades letter to Brayley that described Reades technique of washing silver nitrate paper with infusion of Galls immediately before exposure.  It was unfortunate for historians of photography that Reades letter was misdated in Brewsters publication as 9 March 1839 and not the correct 9 April 1839, and so became a major source of a confused legend that J. B. Reade had invented photography using gallic acid and fixation with hypo not only before Herschels disclosure of the use of hypo in March 1839, but even earlier in 1836 or 1837. [9b]
J. B. Reades interest in gallic acid could not have lasted for more than a few weeks in 1839. He certainly had a knowledge of chemistry good enough to have some notions of his own regarding the interaction of metal salts, organic agents like nutgalls, gelatin and paper, although his work seems to have often been empirical and desultory. He was not usually slow to publish slight work, and the fact that he did not publish his experiments with galls is surely of particular significance and indicates that it gave results less satisfactory than was suggested many years later. It is now obvious that his experiments with gallic acid in 1839 were derived from Herschels original work.
Alfred Smees article on photogenic drawing in the Literary Gazette of 18 May 1839 contains the only reference found to gallic acid in a published work on photography in England during 1839. Smees home was close to the London Institution and he was one of its managers. He could have carried out his experiments on photographic chemistry in the laboratory of the Institution and obviously may have been influenced by E. W. Brayleys lecture. Smee corresponded with Herschel on a number of subjects after 1840, but no evidence is available for any personal contact with Herschel in 1839
The only other published reference to gallic acid is found outside England in connection with photography in 1839 and, in contrast to Smee, of real relevance. It is by G. P. Alexander Petzholdt (181089),  who in 1839 was at Dresden and for many years was Professor of Technology at the University of Dorpat. Although Petzholdt started photographing very early in 1839, his interest in photographic science hardly seems to have survived after 1839 (although it has not been possible yet to research him deeply). He became established as a regular contributor to the German Journal für Praktische Chemie published in Liepzig, and an introductory article by him dated Dresden, dem 6 August 1839 on the chemistry of photography appeared in an issue of the journal printed between September and December 1839.  Although briefly entitled as Ueber Daguerrotypie, Petzholdt did briefly introduce the present state of the art (of which he does in some ways display a superficial acquaintance) as being of two types according to whether the tones were reproduced directly (Daguerres gute method) or reversed (Talbots schlechte method). He cites no sources, and shows no evidence of knowing Herschels work, although a few weeks before he wrote his article Herschels paper of 14 March 1839 had been published in German  in a journal (Frorieps Notizen aus dem ... Natur und Heilkunde) which he can be expected to have read.  In his threepage article Petzholdt mentions gallic acid as one of the organic acids of interest in connection with silver salts:
Es giebt organische Säuren, z. B. Gallussäure, Tanninsäure und mehrere andere, welche beim Zusammentreffen mit salpetersauren Silberoxyd das Silber nur bis zum Oxydul reducien, also einen schwarzen Körper ausfällen, während sie bei passender Behandlung unter Einfluss des Lichtes reines weisses metallisches Silber geben. Bestreicht man demnach Papier mit einer Mischung einer Auflösung genannter Säuren und des salpetersauren Silbers (beide Auflösungen in bestimmter Concentration) und überlässt es der Einwirkung des Lichtes, so erhält man weisses metallisches Silber; lässt man es dagegen im vollkommnen Dunkel liegen, so wird es ganz schwarz. ...
In the 1830s it was widely accepted that an organic combination was required for the light sensitivity of silver nitrate. In a strange way, persons with a knowledge of chemistry in the century before the invention of photography would not have been surprised to have learnt the importance and detailed role given to gelatin by photographic scientists of the twentieth century (as can be amply seen in pages of this Journal of Photographic Science) in connection with the sensitivity of photographic emulsions: indeed they would have quite expected it! Before 1839, such researchers and writers as R. Scanlan (1838), W. T. Brande (1830), John Davy (1828), Tom Wedgwood (1802), and W. Lewis (1763) paid attention to the importance of organic combination of silver salts for sensitivity to light. [9a] In 1839 it can be observed in the writings of many experimenters in photography, including, for example, Alfred Smee, J. B. Reade, Alexander Petzholdt, and John Herschel, that the same concept was prominent in their minds.
The article by Petzholdt in the Journal für Pracktische Chemie is of added interest because it highlights an aspect of the subject that has been neglected. A prevalent feature of the German scientific environment in which Petzholdt educated was a wide attention to organic chemistry. Friedrich Wöhlers synthesis of urea in 1828 was considered the beginning of a new epoch in chemistry. Knowledge of pioneering work on organic acids (including uric acid and the plant tannic and gallic acids) by German and French researchers such as Wöhler, J. von Liebig, and T. J. Pelouze quickly spread. By a coincidence, metals and organometallic compounds were also of interest to these researchers. The idea of using silver salts with organic acids thus came easily and naturally to Petzholdt just as it would to other editorial advisors and contributors to Pracktische Chemie who included Wöhler, Liebig and Franz von Kobell. It is likely that Sir John Herschel had also imbibed the same current discussion of organic acids in the 1830s and is significant for an understanding of many aspects of his work in photographic chemistry.
Today it is naturally accepted that Herschel in his photographic research should have studied the photosensitivity of silver salts, and little surprise (though some admiration) is now expected concerning his pioneering work on dyebleach systems with vegetable dyes and iron blue prints techniques. In contrast, his very early work in 1839 in trying to develop the photographic properties of lead salts, urine and uric acid has since caused some puzzlement. A researcher of wide knowledge like Herschel would obviously be influenced by many disparate works of the past. For example, Otto Tachenius Hippocrates Chymicus of 1666 and Robert Boyles Experiments and Considerations touching Colours of 1664 have an astonishing relevance to Herschels wide researches in photographic chemistry.  But, in addition, contemporary research publications would have a considerable influence on his current trends of thought and few men in England would have been open to influence from German work. Boyle, almost 180 years before, carrying out experiments in consort touching whiteness and blackness used lead precipitation of urine and nitrogeneous (urinous) volatile salts. But such studies and techniques were particularly characteristic and widespread amongst the German organic researchers of the 1830s. Herschels use of gallic acid in association with the photographic properties of silver salts was therefore surely reinforced from the longknown concept of the need for organic combination by a background of contemporary work on organic acids.
As can be seen from his Photographic Memoranda book, Herschel tested the photographic properties of gallate of silver for the first time in the last week of January or the first week of February 1839. A few weeks later, when his paper On the Art of Photography was read at the Royal Society on 14 March 1839, he stated he had tried salts of silver
in which the acid being more volatile ... being of an animal or vegetable origin ... Of these latter several (especially the Gallate and Urate) are still under trial and present remarkable peculiarities) ... impart much greater sensibility to the paper.
His important parenthetical statement about the gallate was one of the only two facts mentioned at the meeting which did not appear (presumably because it was still under trial in the paper when it was published nine days later. Herschel had, of course, mentioned the use of gallate of silver to Talbot in a letter of 28 February 1839, but it was not until Herschels work was pursued further and reported again at the Royal Society that any published reference to gallic acid appeared by Herschel. He sent a copy the printed proofs of this fine paper of February/March 1840 to Talbot before it was actually published in the Philosophical Transactions. But even after this lapse of time, Herschel was not able to explain the effect gallic acid had in combination with silver salts; it was in is own words somewhat problematic. 
Herschels use of gallic acid in 1839 is definite, can be concisely stated and is of prime significance; ie. is in complete contrast with J. B. Reade not only in this regard but in the way the situation has been handled by historians.
Herschels use of gallic acid in 1839 has long been missed in histories of photography in contrast to J. B. Reades role in the introduction of gallic acid to Talbot which received considerable attention in the past because of his historiographically influential appearance in the wellknown lawsuit of Talbot v. Laroche at the end of 1854. In this present article Reade has featured at some unavoidable length due to the inherently confused and poor nature of the evidence, but this amount of attention is therefore unbalanced compared with his real secondary influence on Talbot. It may therefore be surprising that Herschel was also involved, at Talbots insistence, in one of Talbots calotype patent legal cases early in 1854. However, the case was Talbot v. Henderson which was pursued in the Court of Chancery (heard almost entirely in chambers) and Herschels evidence was by written affidavit only. The case did not therefore obtain the same prominence in photographic history as did the conclusive calotype patent lawsuit of Talbot v. Laroche.  Herschel did not make a witness sufficiently partial to either side of the calotype patent dispute in the early 1850s and for several reasons he was not ready to be involved. But involved he was in Talbot v. Henderson:  for his published remark in his paper of 1840 was invoked as an early use of gallic acid prior to Talbots inclusion of this chemical in the calotype patent. It was Robert Hunt, in an affidavit sworn on 22 May 1854 in support of Hendersons side of the case against Talbot, who drew attention to Herschels published use of gallic acid. Talbot wanted Herschel to give him considerable support, but Herschel had some doubts. He was asked it he would just give a reply to Hunts statement, and so on 25 May 1854 (with Talbot almost at his elbow) signed an affidavit that he had originally used gallic acid for the purpose of developing a dormant picture, not then being aware that such dormant picture then existed, but only with a view to increase the sensitiveness of the paper. 
John Herschel brought gallic acid into photographic science in 1839 and this information was communicated to W. H. F. Talbot both directly from Herschel and indirectly through the Rev. J. B. Reade. Herschels idea was base on gallic acid as an organic sensitizer of silver salts. Although by February 1840 he had had some marked success with gallic acid and its compounds, its use was somewhat problematic. 
Talbot experimented again with gallic acid in September 1840. In his Chemical Notebook covering the period from 26 June 1840 to April 1843  the first experimental record of the technique which was later to be called the calotype appears on 2023 September 1840. It is typical of the speculation that arises with this subject that it can now be seen in Talbots notebook that the very words that must have been gallic acid have been carefully cut away from the relevant page with a sharp blade! 
Talbot sealed the title of a patent (No. 8842) for the calotype on 8 February 1841 and enrolled the detailed specification of the technique on 8 August 1841. At the period the title was sealed he also published general articles saying that he had discovered a new process, but waited until the specification was enrolled in August 1841 before publishing the chemical details of the technique. Talbot always concentrated on publishing and patenting specific techniques without discussion of scientific concepts and he was not, for example, interested in writing textbooks to disseminate ideas for future development. The calotype is undoubtedly Talbots principal contribution towards the basic system of photography with which we are familiar today. It is easy to see that the calotype technique contains the now basic system of negative/positive working along with chemical development of the latent image; but it is just as easy to forget that these are not the only elements of the process.
Talbot laid considerable emphasis upon sensitization with silver iodide/gallic acid mixture before exposure. As late as 1844 he still seemed to place importance on this preexposure wash with gallic acid to bring out the picture.  The words latent and develop certainly appear in his first account of the calotype process, but how clearly did he conceive of the development of the latent image? The calotype process presented practical difficulties so that it was still not so widely used as the daguerreotype in the 1840s.  If the calotype was really so basic a beginning to modern photography can the early practical failure of the technique be solely due to the fact that the negative base was paper? Did other photographers fail to read from the published technique an understanding of the development of the latent image?
John Herschel, in contrast to Talbot, always sought to understand the principles involved in the photographic process, and his published articles were therefore always stimulating to other researchers. In the first two years of photographic research he did not, however, observe what has proved to be so remarkable a characteristic of silver salts and the reason why they are so important for the advance of photography; important not simply because they change when exposed to light (for many substances do this) but because development by chemical reduction can be obtained on silver halide crystals after they have received only an extremely low exposure. But it is not surprising that photography should have first to pass through a primitive stage using an exposure printout system before a mature understanding could be reached of latent image formation and a mode of working using chemical development could be established. It is commonly held that the calotype is of absolute significance and Talbots role supreme in the discovery of the latent image and development. Whether or not such assumptions are entirely correct, it is within this context that the present article has attempted to clarify a confused aspect of the prehistory of development in the hope that future studies on the subject can be more straight forwardly and relevantly pursued.
The early history of chemical development and the growth of concepts of the latent image are surely central to the history of photographic science, but even so have been surprisingly neglected. It is well beyond the terms of reference of this article to pursue the theme further. Indeed in the present state of the subject, with a great lack of close studies, no definitive judgement could here be made. As a first step, a reexploration of the discovery and use of developers in the 1840s and early 1850s would be of undoubted value before the more difficult task of assessment and value judgements could be made on the conception of the latent image by researchers in the 1840s.
I am grateful to the Royal Society for allowing me to study archive material in their library and for permission to publish Talbots letter of 28 March 1839 in the Lubbock Correspondence; to the Talbot Museum at Lacock for permission to reproduce Fig. 1; and to the Humanities Research Centre of the University of Texas at Austin, for microfilming Herschel material held there.
1. Wood, R. D., Ann. Sci, 27, 4783 (1971).
2. Wood, R. D., The involvement of Sir John Herschel in the photographic patent case, Talbot v. Henderson, 1854, Ann. Sci, 27, 239264 (1971).
3. For unhappiness at the Royal Society over Talbots papers see footnote 62 on p. 33 of Wood, R. D., Ann. Sci., 27 (1971)
4. Herschels letter to Talbot dated 28 February 1839; MS in Photographic Collection, Science Museum, London, reproduced in Photogr. J., 77, 528531 (1937)
5. John Herschels important Photographic Memoranda book is at the Humanities Research Center of the University of Texas at Austin. Dr. R. S. Schultze was greatly hindered in his valiant and close study of Herschels experimental materials now at the Museum of the History of Science at Oxford because batch numbers only had been written by Herschel on the text papers without information on the chemical treatment given (R. S. Schultze, J. photogr. Sci., 13, 5768 (1965)). The importance of Herschels Photographic Memoranda book is that it provides the key to Herschels experimental batch numbers, but unfortunately was not known to Dr. Schultze.
6. Lubbock Correspondence, Royal Society, London: Letter T20. The particular Thursday Evg on which it was written can be dated with certainty as 28 March 1839 because it was written in reply to a letter of 27 March from Lubbock regarding hypo, and elicited another letter on 30 March in the Talbot Collection, Lacock.
7. Wood, R. D., Brit. J. Photogr., 21 July 1972, p. 611
8. Smee, A., Photogenic drawing, Literary Gazette, 18 May 1839, p.. 314316.
9. The summary in this paper of Reades experiments in photography and his life in the first months of 1839 is integrated from the authors previous studies: (a) J. B. Reade, F.R.S., and the early history of photography, Ann. Sci., 27, 1383 (March 1971); (b) J. B. Reades early photographic experiments recent further evidence, Brit. J. Photogr., 28 July 1972, 119, 644647, 643. Full documentation is therefore not repeated here.
10. Charles Wheatstone and J. F. Royle when refereeing papers submitted by Reade in December 1836 and March 1838 both also noted that his writing was imprecise and wants definiteness (Royal Society: Referees Reports, Vol. 1, Reports 193 and 194). Indeed a slackness is characteristic of Reades letters and communications to journals throughout his life which, misleading unwary readers, certainly has contributed to the continuance of a legend that he was a pre1839 inventor of photography.
11. Letter from J. B. Reade dated 28 February 1839: Royal Society, London, Misc correspondence MC3.15. Published in full in R. D. Wood, op.cit.(Ref. 9a), pp. 3233.
11 bis. [A paper by Robert Rigg was Communicated by the Rev. J. B. Reade at the meeting of 14 March (Proc. Roy. Soc., 4 (183743), 131). Also the Strangers (ie. nonfellows) present at the meeting as listed in the Journal Book of the Royal Society (MSS Vol.48, pp. 482-486) were Lord Cunnyngham, W. H. Smyth, and Dr. Lee, who were, or became, associates of Reade.]
12. Herschel, J. F. W., Note on the art of Photography, The Athenaeum, 23 March 1839, p. 223; Phil. Mag., 14, 365367 (May 1839); Proc Roy. Soc. London, 4, 131133 (183743); Frorieps Neue Notizen aus dem Gebiete der Natur und Heilkunde, 2nd series, 10 (No. 17), cols 260261 (June 1839)
13. Manuscript of J. F. W. Herschels paper read to the Royal Society on 14 March 1839, in St. Johns College Collection (James 510), Cambridge, transcribed by L. Schaaf in History of Photography, vol. 3, pp. 5760 (1979)
14. Letter from J. B. Reade to his brother George, 1 April 1839, in Royal Photographic Society Collection, published by A. T. Gill, Photogr. J., 101, 1013 (1961)
15. Autograph letter, in Royal Photographic Society Collection, written on the morning of Easter Monday, 1 April 1839, from T. S. B. Reade (J. B. Reades father) at Leeds to a younger son George at Guisborough in the North Riding of Yorkshire, where George purchased Hutton Lowcross farm. J. B. Reades letter to his brother (see ref. 14) was written on the back of this letter.
16. Andrew Ross & Co., 33 Regent Street, Statement of Account for W. H. F. Talbot for 1839: Talbot Museum, Lacock, MS. LA39.13
17. Alexander Garden, operative chemist, 372 Oxford Street, London, Statement of Account for W. H. F. Talbot for the period 20 March to 6 December 1839: Talbot Museum, Lacock, MS. LA39.25
18. W. H. F. Talbots Notebook (6 February 1839 25 June 1840), Photographic Collection, Science Museum, London.
19. Copy Memorandum by Mr. Edward Wm. Brayley Junr of the London Institution handed to Messrs Fry and Loxley [Solicitors to Laroche] 14 June 1854, and also sent by him to Mr. W. H. F. Talbot 19 June 1854 (Edward Wm. Brayley, London Institution, 17 June 1854): Talbot Collection, Lacock, MS. LA54.34
20. For the relationship of Brewster and Reade see R. D. Wood (1971), op. cit. (ref. 9a), pp. 18, 7779. It is conceivable that if in late March 1839 Reade had sent a communication to Brewster, rather than directly to Brayley, for publication in the London & Edinburgh Philosophical Magazine, even Brewster could have mentioned Reades experiments to Talbot.
21. The long extract from Reades letter to Brayley, misdated 9 March 1839, appeared on pp. 470471 of article on Photography by Brewster in North British Review, 7, 465504 (1847). A manuscript copy made in 1854 of the complete original letter dated 9 April 1839 has survived in the Talbot Collection, Lacock, MS. LA54.34, and is published in full in R. D. Wood (1972), op. cit. (ref. 9b).
22. J. C. Poggendorfs Biographischliterarisches Hanwörtenbuch zur geschichte der exacten Wissenschaften (Leipzig), Vol 2, pp. 420421 (1863) and Vol. 3 (ii), p.1031 (1898).
23. Petzholdt, A., Ueber Daguerrotypie, Journal für praktische Chemie, 18, 111114 (1839)
24. A brief news item on Petzholdts photography in Dresden appeared in midMarch in Frorieps Notizen aus dem Gebiete der Natur und Heilkunde 2nd series, 9 (19), col 298 (March 1839). This journal (issued twice weekly), reprinted articles in medical and scientific studies indispensable to Dr. Petzholdt.
25. Boyle, R., Experiments and Considerations Touching Colours ... The Beginning of a Experimental History of Colours, London (1664). In the section on Whiteness and blackness Boyle also refers to infusion of nut galls added to iron sulphate (green vitriol) which was a wellknown mixture for gall inks and even used as an invisible ink by writing with only one component and revealing the invisible image by washing with the tannic/gallic infusion, and is a reagent with a long and influential history going back to the ancient World (Nierenstein, M., The early history of the first chemical reagent, Isis, 16, 439446 (1931)). Old links between FeSO4/nutgall and CuSO4/Urine (ammonia) mixtures are relevant to Herschels work.
[[For discussion of invisible gall ink with regard to the latent image see R. D. Wood's later article on The Daguerreotype and Development of the Latent Image: Une Analogie Remarquable]]
26. Herschel, J. F. W., On the chemical action of the rays of the solar spectrum on preparations of silver and other substances ... and on some photographic processes, Phil. Trans., 130, 159 (1840)
27. Wood, R. D., The Calotype Patent Lawsuit of Talbot v. Laroche 1854, published by the author, Bromley (1975).
28. Chancery affidavit sworn by Sir John F. W. Herschel on 25 May 1854 in the case of Talbot v. Henderson. The original affidavit exists at the Public Record Office, London (document No. C31/1048, 733), but was published) probably at Talbots instigation) along with an affidavit sworn by Brewster (but possibly drafted by Talbot) in Notes & Queries, 8 July 1854, pp. 356. See also Wood, R. D., op. cit. (ref. 2). Robert Hunts important affidavit survived only in the manuscript records of the Court of Chancery.
29. Talbot, W. H. F., Chemical Notebook (26 June 1840 to 23 April 1843), Photographic Collection, Science Museum, London.
31. Talbots remarks at the meeting of the British Association for the Advancement of Science at York in 1844 are of particular interest. He was reacting to papers read by Robert Hunt (who was suggesting the use of iron sulphate as a developer of plain silver salt negatives) and Dr. Thomas Woods of Parsonstown, Ireland (who used paper negatives sensitized with ioduret of iron and silver nitrate, without separate postexposure development, but who thought the iron salt continued to act after exposure). Adv. Sci. Brit. Assoc. Report for 1844, Part 2, pp.3637 and 105. See also Wood, R. D., op. cit. (ref. 2), p. 255.
32. Gernsheim, H. and A., The History of Photography, Thames and Hudson, London, 2nd ed. (1969), Ch. 14; Thomas, D. B., The first Negatives, Science Museum Monograph, London (1964); Ostroff, E., The calotype and the photographer, J. photogr. Sci., 26, 8388 (1978).