1891.] Minerale and Petrography. Y 
methods used in his investigations, and of the apparatus employed. 
A thorough discussion of the limitations of ‘his methods covers about 
a hundred pages. After showing that the, results obtained by the 
method used, which is that of the prism, are quite accurate, he records 
his measurements upon calcite, beryl, phenacite, aragonite, barite, 
cordierite, two topaz crystals from different localities, orthoclase, and 
oligoclase, at temperatures up to 330°. In two tables, appended to the 
statements of general results reached in each case, are given the indices 
of refraction for six differently colored rays moving along the several 
axes of elasticity in each mineral, and under different conditions of 
temperature: (1) as measured in warm air, (2) as calculated for a 
vacuum. A third table contains the values of the double refraction 
calculated for a vacuum, and a fourth represents the variations in the 
angle of the prism used produced by different temperatures. Without 
entering into the details of the investigation, it may be said that the 
figures reached by the author are worthy of the greatest confidence, 
and that when differing from those obtained by other observers they 
must be accepted in place of these latter. Among the most 
interesting of the general results obtained are the following : Of the 
minerals examined only barite and aragonite show a refractive index 
decreasing with an increase in temperature. There is a general 
increase in the value of the optical angle and in the dispersion of the 
axes, with increase in temperature. The double refraction increases 
with the temperature only in the case of beryl. Brazilian topaz is the 
only mineral showing a decrease in the amount of double refraction, 1m 
passing from red to violet light. Other points of interest brought out in 
the work are too numerous for mention here. The paper covers ba me: 
dred and eighty-seven pages. In a recent communication arel 
gives a very valuable synopsis of microchemical methods, app uct 
not only to the determination of the constituents of minerals in t in 
sections of rocks, but useful also as aids to the blowpipe in Sepe 
tive mineralogy. His article opens with a few EURA Apr 
on manipulation. Then follows a résumé of the tests that have g1 ese 
most satisfaction in the detection of fifty-nine of the EF as 
are arranged alphabetically, and under each are given the aa A 
for making the tests selected. Wyrouboff ™ proposes a Oe i 
for the explanation of polymorphism, and shows how eaten : a very 
also the explanation of pseudo-symmetry. His paper, T ar sero 
clear exposition of. his views, opens with the account of his 
_ | Neues Jahrb. f. Min., etc., B. B. VIL., P- 434 
® Bull. Soc. Franc, d. Min., XIII., 1890, p. 277- 
Am, Nat.—June.—5. 
