68 REPORT— 1853. 



On the Chemical Action of the Solar Radiations. 

 By Robert Hunt. 



[Second Report.] 



Note. — [In the former Report a division was made between the analytical 

 examination of the solar spectrum by absorbent media, and the chemical 

 results obtained from the spectra which had been thus subjected to absorp- 

 tion. It has been found inconvenient, in the examination of the woodcut 

 illustrations, to have constantly to refer from page to page when comparing 

 the chromatic with the chemical effects. This arrangement has, therefore, 

 been altered in the present Report, and the chemical spectrum is given imme- 

 diately after the description of the luminous spectrum. In all other respects 

 the same order of arrangement is maintained ; the numbers attached to the 

 glasses, &c. remain unaltered, and where new specimens have been introduced 

 they have been numbered in continuation. This remark applies also to the 

 paragraphs, so that reference from one to the other, when required, will be 

 made without difficulty. The uncertain state of the present summer, and 

 the small amount of sunshine with which we have been favoured, has greatly 

 retarded the progress of this investigation.] 



(70). The Chemical preparation employed in the series of experiments 

 which I have now to describe, was the iodide of silver as obtained on the 

 ordinary iodized paper, rendered sensitive by the mixture of gallic acid and 

 nitrate of silver. As, however, I find that nearly every variety of paper, and 

 certainly, every different manipulation, gives rise to an alteration in the scale 

 of sensibility, it becomes important that I should describe exactly the charac- 

 ter of the paper employed. 



A very hard and uniform paper of Turner's was selected ; its surface 

 being beautifully pressed, and presenting a fine ivory character. It was first 

 washed with a solution of sixty grains of nitrate of silver to the fluid ounce 

 of distilled water, and dried ; then with a solution of thirty grains of the 

 iodide of potassium to the fluid ounce of water. After standing for a few 

 minutes, each sheet was placed in a large vessel of water, and allowed to soak 

 for about half-an-hour. After this, being hung by one corner, it was allowed 

 to dry in a warm room ; if the atmosphere was moist, at a short distance 

 from the fire. 



This paper was placed upon the screen on which the spectrum obtained 

 fell, after it had been submitted to the action of the medium under examina- 

 tion. Everything being carefully adjusted, the paper was washed rapidly 

 by a wide flat brush, with the following mixture : — 



Saturated solution of gallic acid 40 drops. 



Nitrate of silver, thirty grains to fluid oz. of water. . 10 drops. 



The action was, in most cases, allowed to continue for a few seconds only, 

 and the image developed itself slowly in the dark, without any subsequent 

 application of the developing fluid. 



(A.) Series of Yellow Glasses (continued). 



(71) 6o. Pure yellow. Colouring matter Carbon. — The visible spec- 

 trum is reduced by the violet and indigo rays ; the orange blends with the 

 yellow, which is consequently much extended (a slight extension arises also 



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