Page:Scientific Memoirs, Vol. 1 (1837).djvu/73

Rh movement is sometimes so marked that the needle nearly resumes its natural position of equilibrium.

If instead of alum other substances were employed as the invariable plate on which the rays issuing from each diaphanous body are successively made to fall, we should still observe differences in the corresponding deviations of the galvanometer; but they would be in general of a less decided kind. It is on this account that we have preferred the alum.

The following are the results, in hundredth parts, of the constant quantity of heat that falls on the plate of alum:

We see that radiations of the same intensity emanating from the diaphanous and colourless bodies contained in the tables pass through the same plate of alum in very different quantities. In the same manner sheaves of luminous rays issuing from different coloured media are transmitted some in greater and others in less proportions by a second transparent substance equally coloured, as the tint of each medium happens to be more or less analogous to that of the invariable substance through which they are to pass.

The calorific rays issuing from the diaphanous screens are therefore of different qualities and possess (if we may use the term) the diathermancy peculiar to each of the substances through which they have passed. The citric acid, the tartrate of potash and soda, and the sulphate of lime transmit rays which pass abundantly through alum; the