182 Mrs Somerville on the Chemical Rays, 4r. 



ginal whiteness, whilst the rest was wholly of a deep brown 

 colour. 



The same experiment has been tried with fine sheets of white 

 mica. Six sheets of common white mica placed on each other did 

 not intercept the chemical rays ; the chloride of silver which they 

 covered, at the end of an hour's exposure to the sun, had become 

 quite brown. The same result was obtained after using a sin- 

 gle plate of mica, which, however, was still thicker than all the 

 others put together. This substance does not appear to present 

 any obstacle to the transmission of the calorific rays. 



These experiments led me at first to suppose that all green 

 substances possessed this property : but I very soon found that 

 this would be drawing too hasty a conclusion ; for, having shortly 

 afterwards tried the experiment with a very large emerald, the 

 green of which was very beautiful, though not very deep, and 

 the thickness of which was at least 0.35 of an inch, I found that 

 it readily transmitted the chemical rays. Thus, the matter 

 which imparts the colour to the green emerald has no action on 

 the chemical rays, whilst that which imparts the same colour to 

 glass and mica has great influence over them. 



Rock-salt, as might be expected, possesses in a high degree 

 the faculty of transmitting the chemical rays. Glass, too, 

 coloured violet with manganese, and very deep blue glass, 

 such as is common in finger-glasses, likewise very readily trans- 

 mit these rays. The alteration in the colour of the chloride 

 of silver very speedily takes place in spite of the interposition 

 of a plate of blue glass of the deepest tint, and nearly a quar- 

 ter of an inch thick. 



Among the various substances which I have tried in these 

 experiments, rock-salt and white glass, as also the blue and 

 violet-coloured glasses, are those which afford the maximum of 

 permeability to the chemical rays ; whilst the green shades of 

 glass and mica present the minimum. Other bodies possess 

 this property in intermediate degrees, and sometimes vary 

 considerably, though the colour is nearly the same. Thus 

 glass of a deep red colour allows very few of the chemical rays 

 to pass, whilst garnet, of an equally deep colour, allows nearly 

 the whole of them to pass. The white topaz, as well the blue, 

 the blue pale beryl, the cyanite, the heavy spar, the amethyst, 

 and various other substances, transmit the chemical rays with 



