Page:Scientific Memoirs, Vol. 1 (1837).djvu/76

64 substances examined by that process to which we have submitted the coloured glasses and the diaphanous colourless bodies.

On these numbers we have two remarks to make: first, that the green tourmaline and the black mica act in a manner analogous to glass of the same colour; second, that the beryl and emerald emit rays equally transmissible by the alum, although the colours of these two kinds of the same substance are different. The same happens to the two kinds of agate. These facts may perhaps be turned to some account by the mineralogist in examining certain coloured substances which belong to the different varieties of one mineralogical species.

We have been hitherto investigating the action of alum on a constant quantity of rays emerging from several diathermanous substances. Let us now reverse the problem and see what will be the effect when these substances are interposed in the passage of an invariable radiation issuing from alum.

In the third column of the following table will be found the results furnished by this class of experiments. It is almost unnecessary to observe that they have been obtained by successively placing the several bodies between the alum and the pile, after having produced in the galvanometer the ordinary deviation of 30° through the first substance. I have placed in the columns after the third the values of the transmissions of the same bodies exposed to the rays emerging from four substances different from alum; namely, sulphate of lime, acid chromate of potash, and green and black glass. The natural values of the calorific transmissions, that is to say, the results obtained under the immediate action of the lamp, are indicated in the second column.