[Journal of Photographic Science, Sept / Oct 1996, 44 (5), 1657]
The concept of a latent image has been widely held to stem from the calotype technique patented in 1841 by W. H. F. Talbot. Although Talbot did lay considerable emphasis upon sensitization with a mixture of silver iodide and Gallic acid before exposure as much as on post exposure development with Gallic acid. These matters relating to the calotype, along with the use of gallic acid in 1839 by Sir John Herschel, J. B. Reade and others, have been previously discussed by the author both in this journal  and elsewhere.  The fact that the earlier daguerreotype technique revealed an invisible image by means of mercury vapour has been given much less historical status, no doubt because the use of vapour appears more remote from later standard use of liquid developers. It is certainly a requirement here to emphasize that the concept of a latent image did indeed enter into photography with the daguerreotype technique in August 1839. But the main purpose of the present short note is to make available to a wider audience the text of an immediate response to the daguerreotype technique showing that an analogy could be made in 1839 between the use of the mercury vapour in that technique with a widely known use of gallic acid. At that time it did seem obvious to make a link between the latent image on a daguerreotype plate to that of invisible inks, to compare mercury vapour with the action of nutgall gallotannins on metallic salts of invisible inks. This clearly adds weight to any assumption that the earliest use of gallic acid to develop photosensitive silver salts on paper is likely to have been derived from a more direct link with practices and concepts of invisible inks.
The daguerreotype was officially announced to the world at the beginning of January 1839 at the Academy of Sciences in Paris. Although the images obtained were shown to many persons during the following months, the technique was kept secret until it was presented on behalf of Daguerre, by François Arago, secretary of the Academy of Sciences. His lecture was presented to a very expectant crowd at the Institut de France on 19 August 1839. Details of the technique, revealing that a silvered copper plate cleaned with nitric acid was iodised, exposed in a camera and an invisible image was then revealed by action of mercury vapour, was published in the general press in Paris the following day of 20 August 1839.  The quality of the reports naturally varied. That of Dr. Alfred Donné (1801-1878) writing for the prestigeous newspaper Journal des Débats  was a substantial account of the lecture. Being widely reprinted in Europe, Dr. Donnés review was of considerable influence both on the immediate spread of knowledge about Daguerres technique then and historiographically. A much poorer report of Aragos lecture by an anonymous reporter entitled Procédé de M. Daguerre appeared in Le Constitutionnel.  Only one-quarter the length of Donnés review, the information on the actual process was limited in several ways. However, Le Constitutionnel substantially supplemented that report on the next day of Wednesday 21 August  when appeared their regular feuilleton revue of scientific affairs. By-lined as by Isid. B, the identity of this man has not been established with certainty, but was possibly Dr. Isidore Bricheteau (1789-1861).  Within the 24 hours immediately following Aragos lecture this reviewer had interviewed Daguerre himself. Perhaps because of this personal contact with Daguerre, Isid.Bs inquiry contains a small amount of information about practical aspects of the daguerreotype technique hardly mentioned so pertinently elsewhere. This shows well, for example, when discussing a not insignificant technicality of judging the correct colour to obtain an even and optimum thickness of silver iodide for the photo-sensitive surface. However, here our major interest in what Isid. B. had to say in Le Constitutionnel on 21 August 1839 lies with his own thoughts concerning development of the latent images obtained on Daguerres silver-iodised plates after exposure in a camera:
After 4 to 10 minutes, according to the period of the day, according to the season, and to the intensity of light, the image of immobile objects from which the lens receives the light, becomes perfectly imprinted on the plate, although this image is yet invisible and only latent [seulement latente]. Plates should be entirely renewed so that new objects do not come to mix their images with any already exposed on the plate. But this image, that is yet, so to say in a state of an unformed chrysalis, what consequently comes to reveal it out of its swaddling clothes? It is the vapour of mercury, from mercury heated to 60° Réaumur. This admirable secret promises immortality for the name of Daguerre, this remarkable man with an exceptional patience to try everything. Previously he had obtained the same effect from oxygen amongst other gases; but the process was also becoming too complicated, too difficult, that it must have been for him an indescribable happy day when, for the first time, he saw the magical effect of the vapours of mercury... A point which cannot be emphasised too much, is that before application of the mercury, there does not exist any distinct image, although these images have already been set-down, and set-down for ever. To bring about better understanding of this state of latency I want to resort to a comparison that has already made M. Daguerre smile, but which I am not afraid to repeat here, because it does not in the least reduce the merit of his discovery. Maybe one has a recollection of a method which schoolchildren and lovers use for sending, without risk of indiscretion, mystery letters where there is no appearance of any writing. The secret of these all white letters is very simple: it consists in wetting a pen in an inkpot containing the pulp and juice of the bulb of alliaceous [onion/garlic] plants. Absolutely nothing is seen of the writing; and it is the same with the image formed by the coating of iodide on the metallic plate. But directly those letters are exposed to a fire, one sees the writing very legibly stand out, the characters becoming as visible as if ink was formed there from nutgalls [authors emphasis]. That is exactly how Daguerres images become visible as soon as the mercurial vapours come in contact. There exists between these two results a remarkable analogy. 
This text is largely self explanatory in showing the author immediately appreciated that the success of the Daguerreotype technique involved the use of mercury vapour to reveal an image invisible et seulement latente. His use of an analogy between this feature of Daguerres unique discovery with a widely familiar action of gallic acid nutgalls on invisible inks is worthy of more discussion.
The multiple but interconnected uses of gallotannins derived from nutgalls have an extremely long and very widespread history going back to the ancient world. The publications of M. Nierenstein in the 1930s provide the best historical perspective of the subject especially as he also issued a useful annotated collection of early original writings.  The chemistry of the gallotannins is complex.  Apart from the eponymous process of tanning, gallotannins have had an intrigingly diverse range of uses centred on their reactions with metal salts. The essential point to be made here is the width of practical familiarity over many centuries with these reactions of plant gallotannins with metal salts. Furthermore it was a familiarity with the reaction as a means to reveal the invisible.
When Sir John Herschel, J. B. Reade, Alfred Smee and Alexander Petzholdt used nutgall gallic acid in conjunction with silver salts in 1839, and W. H. F. Talbot the following year, they did not explicitly cite any source for their notion. Quite simply it would at that time have been entirely familiar and obvious. Twentieth century professional researchers are careful to cite work from which stems their own study. Yet modern citation analysis shows there comes a time when researcher scientists cease to cite the most outstanding discoveries in particular: this obliteration phenomenon  is due to the most unique discoveries becoming quickly incorporated entirely into current cultural inheritance.
In the second century BC, a Greek writer on mechanics and the technologies of war known since as Philo of Byzantium or Philon Mechanicas, described how writing with infusion of nutgalls was invisible, but could be made clearly visible by sponging over with chalcony (ie copperas, green vitriol) being iron sulphate.  Inheritance of such information from the ancient world was probably due most to Plinys influential Natural History where he described the detection of iron sulphate by papyrus treated with infusion of plantgall turning black. The reaction of iron salts with gallotannin derived from nutgalls (where gallic acid is the dominant component) became widely used for inks.  Shakespeare was ready to write a pun of more than one meaning in his Twelfth Night of 1601, Let there be gall enough in thy ink. William Lewis in his Commercium Philosophico-Technicum of 1763 (this work was in English in spite of the title) gave a considerable amount of space to the subject of gall inks and their permanency, as well as revival of faded ink by later treatment with infusion of nutgalls. Such revival of faded ink has been very widely used. Indeed the Public Record Office in London used one percent gallic acid on old parchment or paper documents throughout the nineteenth and first half of the twentieth century. 
A widespread familiarity of the gallotannin reaction with metal salts to reveal the invisible can also be exemplified by the use of these reactions in biology and medicine in many varieties of histochemical studies in which a visible chemical response is required to reveal the site of a specific substance. Heinrich F. Link  in 1807 used the reaction of iron sulphate with tannins and gallic acid to visualise the appropriate cells and vessels in plants.  Gallotannin interactions between proteins and metal salts have been adapted for a wide range of techniques for histochemical light microscopy  in biomedical research and have been extended within the last twenty years to improved ultrastructural visualization of biological tissues treated with metal salts for both transmission and scanning electron microscopy.  Such recent procedures have not been derived from knowledge of photographic development techniques, but rather they stem from a wide and ancient chemical inheritance. Thus it is far from surprising that gallic acid became the prototype developer in the early years of photography after Herschel, Reade, and Talbot used it in conjunction with silver salts on photosensitive paper in 1839 to 1841. In contrast however there exists no substantial clue to the means by which Daguerre arrived at the use of Mercury vapour to reveal an image invisible et seulement latente.
The concept of a latent image has been widely held to stem from the calotype technique patented in 1841 by W. H. F. Talbot, yet it was clearly recognised in 1839 that Daguerres technique produced an image on a plate after exposure to light bien que cette image soit encore invisible et seulement latente. In 1839, an analogy could be drawn between the way the latent image on a daguerreotype plate was made visible with the way invisible inks could be revealed by use of nutgall (gallic acid) infusion. From the ancient world until recent times, there has been a universal familiarity with the reaction of gallotannins with metal salts, in relation to the chemistry of inks, and even more widely with techniques in many fields as a means to reveal the invisible. The universality of these concepts is such that citation analysis of publications over the centuries in which gallotannin reactions with metal salts are used would not reveal the influence under which apparently new ideas were expressed, because the inheritance was so common that it would not be specifically cited. In the case of the daguerreotype technique, the idea behind the mode of revealing a latent image by use of mercury vapour was not so immediately obvious. When Isid. B in 1839 raised the issue of the concepts that might have been involved it is significant that he should naturally think an obvious analogy could be with the use of nutgalls. It is far from surprising that gallic acid became the prototype developer in the early years of photography after Herschel, Reade, and Talbot used it in conjunction with silver salts on photosensitive paper in 1839 to 1841, and that the concepts involved would be unspoken.
 Wood, R. D., Gallic Acid and Talbots Calotype Patent: Part II of J. B. Reade, F.R.S., and the Early History of Photography, Annals of Science, 27, 4783 (1971).
 Wood, R. D., The daguerreotype secret revealed: an annotated bibliography of Aragos lecture of 19 August 1839, (to be published) [article appeared as sections 3 and 4 of A State Pension for L. J. M. Daguerre for the Secret of his Daguerreotype technique published in Annals of Science, Vol. 54 (1997), 489506]
 Académie des Sciences. Séances du 19 Août. Exposition du Daguerrotype...[reported] par Dr. Al. Donné , Journal des Débats, 20 Août 1839, 13.
 Anonymous report Procédé de M. Daguerre, Le Constitutionnel (Paris), Mardi 20 Août 1839, 2.
 Isid. B, Renseignemens sur lexécution des tableaux daguerrotypés , Le Constitutionnel, 21 Août 1839, 12.
 Nouvelle Biographie Générale, 7, 3745 (1853); Dictionnaire de Biographie Français, 7, 284 (1954).
 The original French text from reference 6 is:
après 4 à 10 minutes, selon la lueur du jour, selon la saison, et lintensité de la lumière, limage des objets immobiles dont la lunette reçoit la lumière, se trouve fidèlement imprimée sur la plaque, bien que cette image soit encore invisible et seulement latente. On recouvre de nouveau la plaque, afin que de nouveaux objets ne viennent pas mêler leur image au tableau déjà réalisé. Mais cette image, qui nest encore pour ainsi dire quà letat de chrysalide indistincte, qui donc va la sortir de ses langes et lélucider? Cest la vapeur de mercure, de mercure chauffé à 60 degrés Réaumur. Cet admirable secret qui promet limmortalité au nom de Daguerre, cet homme remarquable ne la du quà sa rare patience à essayer de tout. Déjà il avait obtenu le même effet de quelques gaz, entre autre de loxigène; mais le procédé devenait ainsi trop compliqué, trop difficile, et ça dû être pour lui un indicible bonheur le jour où, pour la première fois, il a vu le magique effet des vapeurs du mercure... Un point sur lequel on ne saurait trop insister, cest quavant lintervention du mercure, il nexiste aucune image distincte, bien que ces images soient déjà tracées, et tracées à toujours. Pour mieux faire comprendre létat latent où elles sont alors, jai besoin de recourir à une comparision qui a déjà fait sourire M. Daguerre, mais que je ne crains pas de répéter ici, parce qu elle ne saurait nuilement affaiblir le mérite de sa découverte. Peut-être sa souvient-on dun precédé dont usent le écoliers et quelques amoureux pour faire parvenir, sans risque dindiscretion, des lettres mystérieuses où ne parait aucune écriture. La secret de ces lettres toutes blanches est très-simple: il consiste à humecter sa plume, en guise dencrier, dans la pulpe et le suc du bulbe de quelque plante alliacée. On ne voit absolument rien d écrit; et il en est de même de limage dessinée sur la couche diode de la plaque métallique. Mais aussitôt que la lettre a été exposée au feu, on voit lécriture se dessiner très-lisiblement, les caractères alors sont tout aussi visibles que si de lencre à la noix de galle les eût formés. Voilà justement comment apparaissent les images Daguerre dès que les vapeurs mercurielles les ont touchées. Il existe entre ces deux résultats une analogie remarquable.
 Nierenstein, M., The Natural Organic Tannins, London: Churchill 1934; M. Nierenstein, Incunabula of Tannin Chemistry, London: Edward Arnold 1932; Nierenstein, M., The early history of the first chemical reagent [nutgall reaction with iron salts], Isis, 16, 43946 (1931).
 More recent work than Nierenstein on the chemistry of Gallotannins has been done by researchers at the University of Sheffield published by R. Armitage, E. Haslam, et.al., in a series of fifteen papers in Journal of the Chemical Society between 1961 and 1967, and Haslam, E., Chemistry of Vegetable Tannins, London: Academic Press (1966).
 Garfield, E., The obliteration phenomenon in Science, and the advantage of being obliterated, Current Contents (Institute for Scientific Information), No.51/52, 57 (22 December 1975).
Exzerte aus Philons Mechanik, B. VII und VIII, vulgo fünftes Buch. Griechisch und Deutsche von H. Diels und E. Schramn, Abhandlungen der Preussischen [Deutsche] Akademie der Wissenschaften zu Berlin 1919, Philosophisch-historisch Klasse No.12, p.79 (Philos Book VIII (on city sieges), iv, 77). Earliest standard publication in Greek and Latin of Philonis de Telorum Constructione liber V edited by Melchisedech Thévenot, Veterum Mathematicorum Opera, Paris 1693, pp. 79104 (on use of nut galls for secret writing p. 102). [British library 59 F1]
 Pliny the elder, Natural History, London: Loeb Classical Library, Heineman 1952, Book 16, xxvi, 1123. Original Latin and translation into English by H. Rackham.
 Mitchell, C. A., and Hepworth, T. C., Inks: their composition and manufacture, London: Griffin (1937), 815, 4192, 12435.
 British Records Association Technical Section Bulletin, No. 16, 423 (1946).
 Allgemeine Deutsche Biographie, 18, 714 (1883). H. F. Link (1767 1851) was at that time Professor of Botany and Chemistry at Rostock. At the same period he was writing a prize essay on the chemical properties of light published in St Petersburg in 1808. He is indeed an outstanding example of how botanists in particular had in the late 18th and early 19th centuries a professional familiarity with the characteristics of the photosensitivity of silver salts.
 Link, H. F., Grundlehren der Anatomie und Physiologie der Pflanzen, Gottingen 1807, pp. 8081. Lillie, R. D., A Histochemical Reaction from 1807: Iron Tannin, Journal of Histochemistry and Cytochemistry, 20, 2956 (1972).
 Chaplin, A. J., Tannic acid in Histology: an Historical perspective, Stain Technology, 60, 219231 (1985).
 Simionescu, N., and Simionescu, M., Galloylglucoses of low molecular weight as mordant in electron microscopy, Journal of Cell Biology, 70, 608633 (1976).
Fresh Nutgalls (popularly called 'Oak Apples') collected from an oak tree in Kent, UK, Oct 2001
Photograph by the author for this online presentation only, not published with the text in 1996
Old Nutgalls collected from an oak tree in Kent, UK, in July 2001
Photograph by the author for this online presentation only