RELATIONS OF LIGHT AND ACTINISM. 369 show that such is not the fact. The blue ray, to quote one example, has freely passed a coloured transparent medium, and yet been entirely deprived of chemical power. It has been obliterated and the chemical power has remained unchanged. (631.) Again, the peculiar fluorescent phenomenon of the spectrum has been spoken of as the chemical rays rendered luminous-made visible; yet, by a block of uranium glass we stop back the fluorescent rays, and allow the permeation of rays which are chemically active. Beyond this, it appears, that we have the power of developing chemical action in the yellow rays, where it was previously masked, by the use of the same medium. (632.) LIGHT, in its pure condition, as light, capable of being decomposed into the primary rays, will not produce chemical change. --- The great exceptions to this are:-organic compounds, which, it has been shown, will change under the influence of Light, separated as far as we possibly can separate it from the chemical principle, and vital organisms, which appear to require the peculiar excitement due to the luminous principle of the solar rays for the support of that great living principle to which growth belongs. No chemical compound of any of the inorganic elements has yet been discovered upon which pure Light produces a chemical change. HEAT, as a radiant force, in some cases does effect a change, but it is of an essentially different character from that produced by actinism. On these grounds, therefore, I still hold to my belief, that the chemical power of the sunbeam, ACTINISM, is a physical agent widely different from LIGHT or HEAT, but associated with these principles in their modes of motion. BB APPENDIX. No. I. HIMLY'S METHOD OF PREPARING THE PROTO- AND PERCYANIDES OF POTASSIUM AND GOLD. [Referred to from page 148.] 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. No. II. ON SOME EARLY EXPERIMENTS IN PHOTOGRAPHY, BEING THE SUBSTANCE OF A LETTER ADDRESSED TO ROBERT HUNT, ESQ., BY THE REV. J. B. READE, M. A., F.R.S. (Referred to from Introduction.) Stone Vicarage, Aylesbury, 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 Lattribute 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 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 spectrum, 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 |