go PETROLOGICAL ABSTRACTS AND REVIEWS 
between 1.5330 and 1.5382 were obtained, the mean value being m= 
1.5370.004 In order to determine if there was any difference in 
the indices of the original balsam used, tests were made upon samples 
submitted by six different firms. Incidentally, in making these deter- 
minations, the values of the indices of refraction of certain minerals 
were obtained. It was found that chalcedony, when occurring in rather 
coarse fibers, is practically uniaxial and has indices a= or o=1.530, 
y or €=1.538. Hydrargillite has values for a and f considerably higher 
than usually given, at least equal to 1.57. In most cordierite,a=1.534 
+0.003, B=1.539+0.003, y=1.541+0.003. Nephelite, so far as the 
indices are concerned, is of two kinds; nephelite from Vesuvius (nephelite 
T) has o=1.5418 and e=1.5378, while elaeolite from Hot Springs, Ark., 
has o=1.5466 and «=1.5417. 
In regard to Canada balsam, the author concludes that the indices of 
the majority of the slides of the Heidelberg collection lie between 1.533 
and 1.541, and in only rare cases do they reach 1. 544 or fall below 1. 533, 
both cases being due to fault of manufacture. Balsam which has turned 
yellow does not always have a high index, but all balsam when exposed 
to the air discolors, becomes brittle, and increases in index. The balsam 
protected by the cover-glass or by a crust of balsam may retain its sticky 
consistency and low index even after 4o years; it is therefore, altered only 
on the surface or at the border of the cover-glass. Commercial balsams 
are so uniform that in the preparation of thin sections the limiting 
values of the index need not fall beyond 1.533 and 1.541, and, with 
practice, should be between 1.534 and 1.540. 
ALBERT JOHANNSEN 
