TALBOT3808

44 Queen Ann S^{t 1}
Feb. 14. 1839

My Dear Sir,

I must congratulate you on your very original and beautiful discovery of the exalting power possessed by certain glasses, which appears to be placed beyond doubt by the specimens which you enclose. I can only explain it, by admitting the hypothesis which you suggest, viz. that certain Solar Rays tend to oxidise & others to deoxidise and that the glass stops the former.

The following extract from Annalen der Physik und Chemie 1835 N^o 8. communicated to me by M^r Wheatstone, ² appears related to the same subject.

“ Professor Hessler, ³ of Gratz, has found that the action of the Spectrum on Chloride of Silver varies with the substance employed for the Prism. The time required to blacken the paper was almost null for water & spirit of wine; 2 minutes for flint glass & 1 minute for Crown glass.”

You mention the difference of hue produced by a transmission of the light thro’ glass: to this I have alluded in my paper to the R.S. §7. on Coloured Paintings on Glass. ⁴ The “indications of colour” which I observed there speak of, were some times sufficiently striking; for instance in copying the figure of a man on painted glass, who was dressed in striped stockings, the stripes sometimes came out a different tint from the rest of the picture.

The adjective “diathermous” proposed I believe, by Melloni, ⁵ is extremely convenient, but unfortunately the substantive corresponding to it is not so easy to hit upon; diathermity, thermousness, or thermaneity, have each their objections – Perhaps transcalescence w^d be the better substantive. But what shall be called the power of transmitting the Chemical Rays? Shall we say that glass is more dichemical than a vacuum, or than atmospheric air?

Baden Powell writes to me from Oxford ⁶ on the practicability, or not, of fixing on prepared paper the bands of colour produced by interference of Solar Rays. I have never tried it, but have no doubt of its being readily practicable. –

Your ingenious method of fixing with the ferrocyanate, ⁷ is different from mine; but whether better or not, I cannot say, time alone can test it properly. The ferroc^te of silver is still present in the dark parts of the paper, mixed with silver or oxide of silver; will it not slowly act upon the latter & whiten it? I mean in the space of a twelvemonth for I have ascertained that these slow Chemical actions go on for a year or two, & produce great changes in the long run. Therefore, unless there exist reasons demonstrative of the Contrary, I should not think the ferrocyanated pictures safe from change, without a year’s experience of them. I know some things will completely whiten the paper blackened with silver in a month (tho’ kept quite dry) which have no apparent effect on it, in a day.

I am sorry that you find a long exposure to sunshine gives a sky blue tint to your f.cyanated paper – Try if washing it with an alkali does not remove this blue tinge.

Having left all my apparatus & chemicals in the country, I have not attempted anything lately beyond one or two little experiments. I am prevented leaving town at present by domestic circumstances, but in a month or two, I hope to be able to produce some much finer specimens than I have hitherto shown; none of which were made with the intention of being exhibited, but entirely as trials & experiments –

Believe me Dear Sir Yours most truly
H. F. Talbot

Sir J. Herschel Bar^t
Slough