Chemical Rays of the Solar Spectrum. 181 



one, however, will understand that the same reserve cannot be 

 imposed upon Mrs Somerville; and I cannot, therefore, with- 

 hold the interesting experiments of this illustrious lady from the 

 Academy, and the public. 



In my experiments, she remarks, I employ the chloride of 

 silver, which Mr Faraday was so kind as to prepare for me, and 

 which, accordingly, was perfectly pure and white. It was liquid, 

 and might be uniformly spread over paper. Although this sub- 

 stance is exceedingly sensible to the action of the chemical rays ; 

 yet as we have no precise means of appreciating the changes of co- 

 lour produced by their action, some uncertainty as to the result 

 might remain were we to compare only those tints which differ 

 but little from each other ; but the results which I shall furnish 

 on this occasion shall be chosen from among those which were, 

 in no degree doubtful. 



A piece of glass, of a light pale green colour, which was per- 

 fectly transparent, and less than ^th of an inch in thickness, did 

 not permit any of the chemical rays to pass ; after exposure for 

 half an hour to a very hot sun, the chloride of silver behind the 

 glass exhibited no change of colour whatever. 



I have repeated this experiment with many pieces of green 

 glass, which differed both in their tint and thickness, and I have 

 always found that they were all nearly impervious so far as the 

 chemical rays were concerned, and even after they had been sub- 

 jected for a much longer period than that above stated to the solar 

 influence. As M. Melloni has already found that glass of this 

 colour arrests the most refrangible calorific rays, by associating 

 his results with mine, we are led to conclude, that glass of this 

 colour has the power of wholly intercepting the most refrangi- 

 ble portion of the solar spectrum. 



Lamina? of mica, of a deep green colour, are also nearly im- 

 pervious to the chemical rays ; however, when they are very thin, 

 and the solar action is continued for a very long time, then it 

 appears that they do not completely arrest these rays. I fixed, 

 with a little wax, to a sheet of paper which was covered over 

 with the chloride of silver, a sheet of pale green mica from Vesu- 

 vius, the thickness of which was not more than the thirtieth of 

 an inch, and I exposed the whole to the rays of a powerful sun ; 

 after a time the sheet of mica being removed, it was found that 

 that part of the paper which it covered retained all its ori- 



