The Columbian Exposition. — Williams. 351 
desired sizes by the Carborundum Company of Monongahela, 
Pa. This new substance is a beautifully crystallized carbide 
of silicon (C Si), accidentally discovered by Mr. E. G. Ache- 
son while watching the action or an electrical furnace upon 
a mixture of charcoal and clay. It is now made in quantity 
by subjecting a mixture of coke and sand to a powerful elec- 
tric current. The result is a mass of tabular hexagonal crys- 
tals of a greenish color, high luster and brilliancy, which pos- 
sess a degree of hardness only inferior to the diamond. Care- 
ful tests by the writer and Mr. G. F. Kunz, showed that these 
crystals would readily scratch a polished sapphire, while a 
wheel composed largely of carborundum will cut rapidly into 
a mass of solid corundum. Practical tests have si own that 
aside from many uses in the arts, this substance is unrivalled 
for the preparation of rock-sections for microscopical study. 
The powders 160 and 220 accomplish the coarser grinding 
very rapidly, while the (5 and 10-minute powders (i. e. mate- 
rials fine enough to remain in suspension 6 or 10 minutes) 
may be used to finish the grinding with unexampled ease and 
expedition. 
Descriptions and photographs of machines for grinding and 
sawing rocks and minerals were exhibited in the Department 
of Liberal Arts by Prof. Dvvight of Vassar College, and by the 
Johns Hopkins University. 
A number of collections of thin sections of rocks and miner- 
als, many of them showing good workmanship, was also scat- 
tered through the exhibition. Thus, in the German University 
exhibit were seen products of the well known firms of Fuess of 
Berlin, Krantz of Bonn, and Voigt & Hochgesang of GOttingen, 
while others of the same kind were placed in the German optical 
exhibit in the Electrical building. The admirable work of Ivan 
Werlein of Paris was shown in the French department of Lib- 
eral Arts and in the Harvard University (Bigelow) collection 
of agates. American work was exhibited by the N. Y. state 
Museum of Natural History ;tt Albany, by Prof. Dwight of 
Poughkeepsie. and by Prof. Wolff of Harvard. It was also 
shown by the U. S. Geological Survey in the Government build- 
ing. Richards & Co>'s exhibit in the gallery of the Mining 
building contained a considerable display of rock and mineral 
sections by Max Wasserschleben of Giessen, but perhaps 
the best exhibit of this kind to be round anywhere in tin- 
