220 Analyses of Books, [Sispr. 
crystalline structure more evident. The specific gravity of the un- 
annealed and annealed drops was found to be nearly the same, 
3°276, allowance being made for the cavities contained in the un- 
annealed drops. These vacuities are occasioned by the contraction 
of the internal parts of the drop while cooling. They disappear 
when the drop is heated to redness. It appears, then, that heat 
produces a crystalline structure in glass, which vanishes as the glass 
cools. 
‘2. Deseription of a new Instritment for performing mechanically 
the Involution and Evolution of Numbers. By Peter M. Roget, 
M.D.—This instrument consists in a very convenient and inge- 
niously contrived sliding rule, which must be useful in a great 
variety of cases. rade 
3. Experiments on the Depolarization of Light, as exhibited ly 
various Mineral, Animal, and Vegetable Substances, with a refer- 
ence of the Phenomena to the general Principles of Polarization. 
By Dr. Brewster.—In this paper Dr. Brewster gives a list of 58 | 
substances, animal, vegetable, and mineral, which depolarize light; 
and of 53 substances, which have no effect in depolarizing light. 
He then gives what he calls a theory of the depolarization of light. 
The various modes in which bodies depolarize light may be reduced 
to seven. 1. When the crystal possesses neutral axes, and forms 
two images which are capable of being rendered visible, as in cal- 
careous spar, topaz, &e. In this ease he shows that the apparent 
depolarization of the pencil is nothing more than the polarizing of 
it in anew plane. 2. When the crystal possesses neutral axes, and 
exhibits ony a single image, as in the human hair, and various 
transparent films. ‘This he considers as exactly the same with the 
first case, excepting that the two images formed by the human hair, 
&e. being ftaduced by the same, or nearly the same, refractive 
power cannot be rendered visible by any contrivance. 3. When the 
erystal has no neutral axes, but depolarizes light in every ‘position, 
as in gum arabic, caoutchouc, tortoise-shell, &c. These bodies are 
composed of thin plates lying above each other. Each of these 
plates possesses neutral axes, and depolarizing axes. But as these 
different axes do not coincide with each other in the different plates, 
the consequence is, that the compound body depolarizes in every 
direction. 4. When there is an approach to a neutral axis, asin 
old-leaters’ skin, &c. In this case the body. is composed of thin 
films, like the preceding; but the neutral axes of each are nearly 
coincident. 5. When the crystal depolarizes or restores only a part 
of the polarized image, as in a filme from sew weed, and a Jilm from 
the'partan’ (erab). He considers that this’ cuse is owing: to the bodies 
which possess this: property being partly crystallized and partly un- 
erystallized. 6. When the crystal depolarizes luminons sectors of 
hebulous light, as the oi] of mace. How the halo in this’ case is 
produced, lie does not attempt to explain: but he conceives that it 
necessarily follows from the phenomena that there. are two halos or 
nebulous images; the one lying exactly above the other, and having 
