$40 M. MELLONI ON THE POLARIZATION OF HEAT. 
coloured glasses to act in this case like the green and opake-black-glasses 
of the preceding table, and vice versd. 
Moreover, the white, red, orange, yellow, blue, indigo, and violet 
glasses all produce very nearly the same alteration in the value of the 
index of polarization of the green tourmalines: and we have just ob- 
served that these glasses have a very marked effect on the pale yellow 
tourmalines. This effect, varying in different systems, but constant in 
each particular system through the whole series of plates, is perfectly 
analogous to the uniformity of the ratios which, notwithstanding the 
changes of intensity, are observed to exist between the quantities of 
heat transmitted by the same glasses (coloured and uncoloured) when 
successively exposed to the rays emerging from the several kinds of 
screens*. We are thus brought back once more to one of the fore- 
going conclusions, namely, that the colouring substances have no power 
of elective absorption with respect to the rays of the calorific stream 
which passes through the glass. 
Before we conclude our observations on the effect produced by the 
screens in the index of polarization of the tourmalines we shall make 
some remarks on the effects produced by the variation of thickness in 
the interposed substance and by the solution of salts in water. 
On inspecting the second table it is easy to see that the influence of 
each substance in augmenting or diminishing the index of polarization 
is more powerful in proportion to the greater thickness of the substance. 
Thus, water reduced to a layer of O™™*74 in thickness causes the index 
of the pale yellow tourmalines to rise from 22 to 36, while a layer of 
g™™ yaises it to 67. On the other hand, a plate of dark glass 0™™81 
in thickness, which causes the same index to descend from 22 to 4, 
would reduce it even to 1°5 if its thickness were about 2™™. All this . 
accords with the experiments of successive transmission, which show 
that the calorific stream emerging from a given substance becomes more 
simple, or, if the expression be preferred, more purified in proportion 
to the greater thickness of the substance through which it has passed. 
We find this to be the case with white light also in penetrating coloured 
media. : 
Tartaric acid, rock salt, and alum dissolved to saturation in water 
make no sensible change in its action on the index of polarization of 
the tourmalines. We have already shown, in a preceding memoir, that 
alum and rock salt (which, of all perfectly diaphanous and colourless sub- 
stances, are those that possess the maximum and the minimum of diather- _ 
mancy ) do not by being dissolved in water affect the diathermanous power 
of this liquid*. The sensible equality of the action of these solutions 
and that of pure water on the natural index of polarization of the tour- — 
* Ann. de Chim. et de Phys., tom. ly. p. 55. 
