1801.] Mineralogy and Petrography. 143 
water, and above this temperature is decomposed. Just below 260° 
the hydrate is biaxial, while above this temperature the anhydrous 
residue is uniaxial. Other zeolites yield similar results, These lead 
to the conclusion that they all consist of a nepheline, pyroxene, or 
feldspar-like silicate, combined with meta- or orthosilicic acid, and 
also an amount of water varying with the temperature. The crystal 
water may be driven off at high temperature, and taken up again at a 
lower one, and the various hydrates obtained by the successive steps 
may possess different crystallographic properties. After a certain 
amount of loss the minerals refuse to part with more water, which is 
regarded as chemically combined with silica in the silicic-acid portion 
of the combination. The author determines incidentally the solubility 
of several of the zeolites in different solvents, and concludes his paper 
with a table giving the supposed composition of the members of the 
group. Heulandite is represented as CaAl,Si,O,,+2H,SiO,+34q ; 
natrolite as Na,A1,Si,O,+H,SiO,, etc. Messrs. C. and G. Friedel,” 
by the action of lime on mica in the presence of calcium chloride, 
obtained small crystals of anorthite, and by the action of soda and 
sodium sulphate on the same mineral produced little prismatic crystals 
of a substance differing from nosean in the addition of two molecules 
of water. 
Physical Mineralogy.—The discussion as to the caùse of optical 
anomalies in uniaxial crystals has: received another addition in a late 
article contributed by Martin," in which the writer attempts to show 
that the Mallard theory with respect to these phenomena is faulty. 
Mallard believes that the crystals are pseudo-uniaxial ; that they consist 
of several twinned individuals, which by their combination build up a 
form possessing a geometrical symmetry of higher grade than that be- 
longing to its individual constituents. Martin has examined several 
organic compounds, and is thereby led to the conclusion that in these 
the anomalies are due to strain or pressure exerted on some parts of the 
crystal by the more rapid growth of other parts. It is well known that 
in many crystals a skeleton is formed first in the act of crystallization, 
and that this skeleton is subsequently filled in by the deposition of 
material within its arms. ‘The skeleton thus grows faster than the in- 
terstitial substance, and exerts in this latter a strain whose effect is 
exhibited in the anomalies. Other important thoughts are brought 
Out in the investigation, which appears to have been conducted in a 
_ ™ Bull. Soc. Franc. d. Min., XIII., 1890, p- 233- 
1 Ib., p. 238. : 
Neues Jahrb. f. Min., etc., B.B. VIL., p-1. 
-—4. 
Am. Nat.—February 
