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Document number: 3786
Date: 02 Feb 1839
Recipient: TALBOT William Henry Fox
Author: WHEATSTONE Charles
Collection: British Library, London, Manuscripts - Fox Talbot Collection
Collection number historic: LA39-005
Last updated: 9th April 2008

20 Conduit St <1>

Feby 2d 1839

My dear Sir

According to my promise I send you a few notices respecting the action of light on chloride of silver and other substances which I hope may be of use to you in your future photographic experiments.

Professor Hessler, <2> of Gratz, has found that the action of the solar spectrum on paper covered with gum water and powdered over with chloride of silver varies with the substance employed for the prism. The time required to blacken the paper was almost nul for water and spirit of wine; from 12 to 13 minutes for oil of turpentine and cassia; 2 minutes and 3 seconds for fluid glass, and one minute 5 seconds for crown glass. The maximum of chemical effect was found, for the spectrum produced by the spirit of wine prism, in the violet near the blue; for that of water, in the middle of the violet; for that made with oil of cassia, 23 lines beyond the extreme violet.

(Annalen der Phys & Chem. 1835. No 8) <3>

M. W. Artus <4> of Jena states (Journ für prakt. Chemie Vol 8) that among bodies on which light exerts a powerful action the iodide of mercury is one of the most remarkable. Exposed to the solar rays it becomes immediately a deep olive green; but if it be previously gently heated in a sand bath it shows itself almost indifferent to the action of light.

It is asserted by Marbach <5> in his Encyclopedia art: Licht that, a solution of starch in boiling water coloured with iodine completely loses its colour when exposed to the sun's rays. M. Lassoigne <6> has made with this substance, which he calls an iodide of amidine, some very interesting experiments. He finds that the colour becomes fainter in proportion as the temperature is raised and completely disappears at 80º or 90ºC. leaving the water entirely deprived of all colour and with the ordinary limpidity and transparency. On cooling either gradually or suddenly it gradually deepens and finally becomes as dark as at first. The experiment may be repeated several times in succession, but if the liquid be subjected to boiling heat for a few minutes it preserves its transparency while cooling, and never reassumes its former colour. Persoz and Payen <7> have repeated these experiments with complete success and have added to them.

The action of light on iodide of amidine is obviously very different to that on chloride of silver, the calorific and not the chemical rays produce the effect. It was with this substance and some others indicated in Chevreul's <8> admirable essay that I had intended to make a few experiments when I first read an account of what Daguere <9> [sic] had accomplished.

Yours very truly

C. Wheatstone


1. London.

2. Ferdinand Hessler (1803-1865), served as professor of Chemistry and Mathematics at the University Graz from 1830-1835, and after at the Karl-Ferdinands University, Prague.

3. Annalen der Physik und Chemie.

4. Wilibald Artus, 'Darstellung des schwefelcyankaliums und dessen verhalten zu dem Strychnin', Erdmann's Journal für Praktische Chemie, later Journal für Pracktische Chemie, v.8, 1836, pp. 252-256.

5. Christian August Hermann Marbach (1817-1873) was a Professor Extraordinary at the University of Breslau from 1861 to 1873; he published on light and magnetism.

6. Jean Louis Lassaigne (1800-1859) chemist.

7. Jean-François Persoz (1805-1868) chemist, and Anselme Payen (1795-1871) chemist.

8. Michel Eugene Chevruel (1786-1889).

9. Louis Jacques Mandé Daguerre (1787-1851), French artist, showman & inventor.

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