116 Grenville Cole—Artificial Perlitie Structure. 
While preparing microscopic sections in the Geological Labo- 
ratory of the School of Mines, I accidentally touched with wet 
fingers the square of glass on which a specimen for grinding was 
becoming set in stiff Canada balsam. This balsam had been some- 
what over-heated, and had been left to harden for about a minute. 
The sudden cooling consequent on the contact developed a series of 
cracks throughout its thicker portions, and these in all their details 
correspond to.the perlitic structure. 
Fic. 1.—Perlitic Structure pro- Fic. 2.—-Section of Perlite 
duced in Canada balsam from Hlinik, near 
by rapid cooling. Schemnitz. 
The formation of the secondary or curved contraction-cracks has 
indeed extended farther than in many natural perlites, four or five 
segments within one another being not uncommon in this artificial 
specimen. A series of fairly rectilinear joints separates the groups 
of curves, the general character being best shown by the accom- 
panying figure. A section of a natural perlite, drawn to the same 
scale of enlargement, is given for purposes of comparison. 
Subsequent experiments show that the rectilinear series may be 
produced easily enough by rapid cooling; but it is only when the 
conditions allow a farther contraction to take place that the sets of 
curves are formed between them. The procedure has been in later 
instances to lay the thoroughly-heated slide on a china plate con- 
taining a thin layer of water, taking care that the balsam itself 
remains exposed to the air alone. The cracks speedily develope, 
being in some cases exceedingly regular in their characters. 
As usually happens, no intentionally-produced. result has as yet 
approached in completeness that accidentally attained; but I have 
no doubt that, after some attempts, a good typical specimen of 
perlitic structure might be procured. 
The frequent superior breadth and blackness of the concentric 
segments may perhaps be attributed to their being portions of actual 
spheres, and in several places their two bounding edges require a 
different adjustment of focus under the microscope. ~ 
