x 
_ and that the conclusions reached by him comprehend no new notio 
the values of the optical constants, and the fifth contains a 
follows: The rapidity of solubility is equal along equivalent € 
18 Miner. u, Petrog. Mitth., X1., 1890, P- 349 = 
144 The American Naturalist. [Febr 
careful and conscientious manner. ‘W yrouboff,® in a reply to N 
tin’s article, states that the latter’s results differ but little from his ox 
The writer last referred to (W.) has recently 16 completed a series 
of experiments on circularly polarizing substances, by which he seems 
to have shown that the peculiar property of these bodies is due to their 
structure, which is described by Mallard as an irregular piling of very _ 
small biaxial plates. In this way a high grade of symmetry is imitated, 
while the plates are really of a low grade. He also adds a nineteenth 
substance to the list of rotatory polarizing bodies, viz., (NH,)LiS' 
which is apparently biaxial and positive. The effect.of tempe 
upon the optical and crystallographic constants of prismatic su 
has been thorougly investigated by Schrauf,” who records his results | 
an excellent paper of fifty-nine pages. The first part discusses the 
values of the interfacial angles at different temperatures. The 
is confined to refractive phenomena, such as the refractive index 
different wave lengths. The third and fourth contain calculati l 
of the relations existing between the refractive indices 
length of the transmitted light, temperature, and other facto 
concludes with remarks on the constancy of the refractive and d l 
ive power, and upon the crystal form of prismatic sulphur.——A paper 
by Becke ! on the etching of fluorite is a remarkable exhibit of caret! 
and painstaking work in this branch of physical mineralogy. The 
author has subjected both natural and prepared faces of crystals 
various localities to the action of acids and alkalies of various 
and at different temperatures, and has studied the results 
The symmetry of the figures obtained indicate a tetragonal 
for the mineral. Anomalous figures on some crystals, found oniy 
planes that show double refraction, are explained as due to the 
of growth. Many new ideas are gathered from the study, one 
‘most important of which is embodied in a restatement of t 
symmetry of etched figures. ‘These possess the symmetry 0 
on which they occur only when this is a natural one free from 
vicinal planes, etc. Experiments on the solubility of the! 
different directions lead to the expression of a law of s 
3 Bull. Soc. Franc. d. Min., XIII., 1890, p. 94- 
m Ib., p. 215. i ~- 
11 Zeits. f. Kryst., XVIIL., 1890, p. 114. 
