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The Protocyanide of Potassium and Gold is best obtained by dissolving seven parts of pure gold in nitro-muriatic acid, precipitating with excess of ammonia, washing the fulminating gold that is formed, and then putting it into a hot solution of six parts of the cyanide of potassium in water. The liquid is decolorised and ammonia disengaged. From the concentrated solution the double salt crystallizes in beautiful prisms.

The Percyanide of Potassium and Gold is very easily obtained when thirty-five parts of gold are converted into chloride of gold, as neutral as possible, and the aqueous solution of that salt is united gradually with a hot solution of forty-six parts of cyanide of potassium. The liquid loses colour, and on cooling of the concentrated solution, the double salt separates, in large prisms, which, on exposure to the air, or in vacuum, become milk white, giving off their water of crystallization.



Stone Vicarage, Aylesbury,

February 13. 1854. MY DEAR SIR, -In giving you the information you require respecting my early researches in photography in 1836 and following years, I may assume that you are already aware, from my letter to Mr. Brayley of March 9, 1839, and published in the “British Review” for August, 1847, that the principal agents I employed, before Mr. Talbot's processes were known, were infusion of galls as an accelerator, and hyposulphite of soda as a fixer.

I have no doubt, though I have not a distinct recollection of the fact, that I was led to use the infusion of galls from my knowledge of the early experiments by Wedgwood. I was aware that he found leather more sensitive than paper ; and it is highly probable that the tanning process, which might cause the silver solution to be more readily acted upon when applied to the leather, suggested my application of the tanning solution to paper.

In your own history of the photographic process, you say “the discovery of the extraordinary property of the gallic acid in increasing the sensibility of the iodide of silver was the most valuable of the numerous contributions which Mr. Talbot has made to the photographic art.” It is nevertheless true, as stated by Sir David Brewster, that “the first public use of the infusion of nut-galls, which is an essential element in Mr. Talbot's patented process, is due to Mr. Reade," and in my letter to Mr. Brayley I attribute the sensitiveness of my process to the formation of a gallate or tannate of silver. I need scarcely say, that among various experiments I tried gallic and tannic acid in their pure state, both separately and mixed; but the colour of the pictures thus obtained with the solar microscope was at that time less pleasing to my eye than the rich warm tone which the same acids produced when in their natural connexion with solutions of vegetable matter in the gall-nut. This organic combination, however, was more effective with the solar microscope than with the camera, though the lenses of my camera were five inches in diameter. It is probable enough that the richer tone was due to the greater energy of direct solar rays. In using the solar microscope, I employed a combination of lenses which produced a convergence of the luminous and photogenic rays, together with a dispersion of the calorific rays, and the consequent absence of all sensible heat enabled me to use Ross's cemented powers, and to make drawings of objects inclosed in Canada balsam, and of living animalcules in single drops of water. The method I employed was communicated to the Royal Society in December, 1836, and a notice of it is contained in the “ Abstracts.”

You inform me that some persons doubt whether I really obtain gallate of silver when using an infusion of gall-nuts, and that one of Mr. Talbot's friends raises the question. It is sufficient to reply, that though gallic acid is largely formed by a long exposure of an infusion of gall-nuts to the atmosphere, as first proposed by Scheele, yet this acid does exist in the gall-nut in its natural state, and in a sufficient quantity to form gallate of silver as a photogenic agent; for M. Deyeux observes, that “ when heat is very slowly applied to powdered gall-nuts, gallic acid sublimes from them, a part of which, when the process is conducted with great care, appears in the form of small white crystals.” M. Fiedler also obtained gallic acid by mixing together a solution of gall-nuts and pure alumina, which latter combines with the tannin and leaves the gallic acid free in the solution ; and this solution is found, on experiment, to produce very admirable pictures. But what is more to the point, Mr. Brayley, in explaining my process in his lectures, showed experimentally how gallate of silver was formed, and confirmed my view of the sensitiveness of the preparation. It is therefore certain that the use of gallate of silver as a photogenic agent had been made public in two lectures by Mr. Brayley at least two years before Mr. Talbot's patent was sealed.

I employed hyposulphite of soda as a fixer. Mr. Hodgson, an able practical chemist at Apothecaries' Hall, assisted me in the preparation of this salt, which at that time was probably not to be found, as an article of sale, in any chemist's shop in London. Sir John Herschel had previously announced the peculiar action of this preparation of soda on salts of silver, but I believe that I was the first to use it in the processes of photography. I also used iodide of potassium, as appears from my

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letter, as a fixer, and I employed it as well to form iodide of lead on glazed cards as an accelerator. Iodide of lead has of itself, as I form it, considerable photographic properties, and receives very fair impressions of plants, lace, and drawings when placed upon it, but with the addition of nitrate of silver and the infusion of galls, the operation is perfect and instantaneous. Pictures thus taken were exhibited at the Royal Society before Mr. Talbot proposed his iodized paper. The microscopic photographs exhibited at Lord Northampton's in 1839 remained in his lordship's possession. I subsequently made drawings of sections of teeth; and one of them, a longitudinal section of a tooth of the Lamna, was copied on zinc by Mr. Lens Aldous for Owen's “Odontography.” I may say this much as to my own approximation to an art, which has deservedly, and by universal consent, obtained the name of Talbotype.

Sir David Brewster, in his “History of Photography," passes immediately from the experiments of Wedgwood to those of Talbot; but the Transactions of the Royal Society," to which my friend Mr. Gravatt has directed my attention, will enable us to insert, if not a chapter, at least a very pregnant parenthesis. The Bakerian Lecture, in 1803, by Dr. Young, who never touched a subject without leaving his mark upon it, contains a highly interesting and original experiment on the photographic representation of the invisible chemical rays beyond the blue end of the spectrum. This experiment does not happen to be recorded in the first edition of your “Researches on Light;" but no one will refer to it with greater pleasure than yourself, not only because it is the first photographic analysis of the spectrum, but also because it has the higher merit, even as it stands alone, of being the one sufficient fact which establishes the consummation so devoutly looked for, at the conclusion of your work, from the persevering accumulation of facts only; for it is in itself a simple and demonstrative proof, to use the words of Dr. Young, of the general law of interference, and, in your own words, “reconciles the chemical action of the photographic force, energia, with the undulatory theory of light." Dr. Young's experiment forms the conclusion of his lecture, and is given in the following terms:-“The existence of solar rays accompanying light more refrangible than the violet rays, and cognisable by their chemical effects, was first ascertained by Mr. Ritter; but Dr. Wollaston made the same experiments a very short time afterwards, without having been informed what had been done on the Continent. These rays appear to extend beyond the violet rays of the prismatic spec. trum, through a space nearly equal to that which is occupied by the violet. In order to complete the comparison of their properties with those of visible light, I was desirous of examining the effect of their reflection from a thin plate of air capable of producing the well-known rings of colours. For this purpose I formed an image of the rings, by means of the solar microscope, with the apparatus which I have described in the Journals of the Royal Institution, and I threw this image on paper dipped in a solution of nitrate of silver, placed at a distance of about nine inches from the microscope. In the course of an hour, portions of three dark rings were very distinctly visible, much smaller than the brightest rings of the coloured image, and coinciding very nearly in their dimensions with the rings of violet light that appeared upon the interposition of violet glass. I thought the dark rings were a little smaller than the violet rings, but the difference was not suffi

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ciently great to be accurately ascertained; it might be as much as sth or oth of the diameters, but not greater. It is the less surprising that the difference should be so small, as the dimensions of the coloured rings do not by any means vary at the violet end of the spectrum so rapidly as at the red end. For performing this experiment with very great accuracy a heliostate would be necessary, since the motion of the sun causes a slight change in the place of the image ; and leather impregnated with muriate of silver would indicate the effect with greater delicacy. The experiment, however, in its present state is sufficient to complete the analogy of the invisible with the visible rays, and to show that they are equally liable to the general law (of interference), which is the principal subject of this paper."

It detracts nothing from the greatness of Dr. Young to say, that although the philosophy of this experiment is permanent truth, yet the spectral image of it soon faded away. Photography was not then, at the beginning of the century, an art as permanent as it is elegant and useful. Little was wanted to make it so, but it hung fire for nearly fifty years, till Talbot supplied that little.

I have just learnt from Admiral Smyth, that his friend Dr. Peacock, the Dean of Ely, has for the last seven years been engaged on a Life of Dr. Young; and when the work appears, we shall have a more accurate knowledge of the man who was unquestionably the Newton of his day. Like his illustrious predecessor, he was a pioneer in the philosophy of light; and, as we have seen, by a single photographic experiment, overlooked hitherto by us all, has shown a perfect analogy between the undulations of the visible and invisible rays. Had he happened to head his chapter, as Wedgwood does, “On a Method of taking Pictures by the Agency of Light on Nitrate of Silver,” his name and place would have been duly marked; but because theory, and not experiment, was the great point before him, the philosophical photographer is overlooked by the practical one.

Dr. Young's propositions are, that radiant light consists in undulations of the luminiferous æther, that light differs from heat only in the frequency of its undulations, that undulations less frequent than those of light produce heat, and that undulations more frequent than those of light produce chemical and photographic action,-all proved by experiments.

You close your own “Researches on Light” by proposing the following questions as of the greatest importance for future investigation: -“ Is energia absorbed by material bodies? Does it influence their internal constitution? Is it radiated from bodies in the dark, or at all concerned in the production of any of those changes which have been attributed to dark rays?"

Dr. Young's hypothesis seems to anticipate your questions, and almost to answer them in their order. He says, “All material bodies have an attraction for the ætherial medium, — by means of which it is accumulated within their substance, and for a small space around them in a state of greater density, but not of greater elasticity.” (Bakerian Lecture, 1801.) Hence be considers material bodies to have within them latent light, latent heat, and latent chemical force, or “energia” (which is, in his opinion, a particular condition of the ætherial medium); that the luniinous, calorific, and chemical phænomena are exhibited under two

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