44 Queen Ann St <1>
Feb. 28th
Dear Sir
As there appeared to be some little difference of opinion between us the other day respecting the iodide of silver, I enclose specimens <2> of my manufacture. According to me there are 3 different things confounded together hitherto under that name; the first, <3> is very sensitive to light & is what you make use of in your experiments.
The second, <4> a pale yellow iodide, is insensible to light: but on the other hand, sensitive to heat. If you try this specimen before a hot fire, (an almost scorching heat is required) you will see it turn to a full & bright yellow, & then back again as soon as it is cold.
The third, <5> or white iodide, is insensible both to light, & heat. The specimen I send is not a good one since there is a patch of the yellow kind on it, which is sensitive to heat.
Believe me
Yours very truly
H. F. Talbot
Notes:
1. 44 Queen Ann Street: London home of the Mundy family and a frequent base for WHFT.
2. No enclosure.
3. The sensitivity to light of silver iodide depends on the manner of its precipitation from solutions of silver nitrate and potassium (or other) iodide, and the exact workings of these compounds were not understood in WHFT’s day. In this first case, the silver nitrate was precipitated in excess, making the silver iodide very light sensitive.
4. In this second case, where the potassium (or other) iodide is precipitated in excess, the silver iodide was insensitive to light. This property was put to use by WHFT to ‘fix’ some of his photogenic drawings. When heated, silver iodide will change its crystalline lattice from hexagonal to cubic, resulting in the colour change noted.
5. This third case remains unidentified and may not even be a silver iodide but some more complex salt, impervious to heat.