n
inch thick. Lathes employed in polishing minerals require to be
provided with conical spindles, so that the wear, due to grit and
emery dust getting on them, may be readily taken up. The grinding
wheel may be either horizontal or vertical; the former has the
advantage that the mineral can be held in either hand; with the latter
only the right hand can be employed, and that in an awkward and
tiresome position. Mr. Julian then referred briefly to the kinds of
emery, its preparation by elutriation, etc., and cautioned operators
against using rouge or tin putty powder in polishing rock sections,
although they may be employed in polishing certain minerals and gems.
The object of making the rock sections being to study their
constituents and determine what minerals enter into their composition,
it is important that no foreign substance, liable to adhere to the
specimen and to be mistaken for one of its ingredients, be placed on
the section while grinding. Lastly, the minerals are mounted on glass,
with or without covers, by means of Canada balsam. Square glasses are
to be preferred to the long and narrow strips, usually employed, as
less liable to break in the center, and more easily revolved on the
stage of a microscope.

Mr. L.H. Landy then exhibited, by means of the gas microscope, several
beautiful rock sections, both American and German. The same gentleman
also showed the effect of passing polarized light through certain
crystal sections, the black cross and rainbow-hued rings revolving
like so many wheels as the polarizer was turned.

At the conclusion of this brilliant exhibition, Dr. P.T. Austen made
some remarks on

LABORATORY MANIPULATIONS.

The points referred to were the apparently unimportant details which
often contribute so much to the ease and pleasure of working. First,
the use of square pieces of felt, such as are used under beer glasses
in saloons, for setting hot beakers and flasks on to prevent chilling
and consequent cracking. Second, in crystallizing substances for
examination under the microscope; one watch glass is placed upon
another with the substance between them, and the upper glass filled
with ether, the cold produced by its evaporation hastening the
crystallization. Third, removing precipitates and solid matter from
flasks, by heating to boiling, and inverting in a vessel of water.
Fourth, crystallization by gradual dilution. Fifth, filter paper
without ash. In German laboratories it i