"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 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."