Scientific American, Volume XXXVI., No. 8, February 24, 1877 by Various, is part of the HackerNoon Books Series. You can jump to any chapter in this book here. New York Academy of Science
NEW YORK ACADEMY OF SCIENCES.
On Monday evening, January 29, 1877, a meeting of this Academy was held at the School of Mines, Columbia College, Dr. J.S. Newberry, President, in the chair. Mr. A.A. Julian, A.M., read a paper on the
PREPARATION OF ROCK AND MINERAL SECTIONS FOR MICROSCOPIC STUDY.
The speaker described in detail the various operations, exhibited the different kinds of apparatus employed, showed the operations, and exhibited the finished sections. In some rocks a thin chip can be broken off, others require to be sawn, and for the latter purpose the diamond saw is best. Having obtained the chip, it is first polished on one side, then cemented to a little square of glass, and the other side polished in the same way. The sections must not be too thick, nor too thin; they are usually made from a hundredth to a thousandth of an 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 is customary to dissolve out the mineral matter from white filtering paper by washing in dilute hydrochloric and hydrofluoric acids. Sixth, the use of infusorial silica for drying purposes. Being very porous, it will absorb five times its own volume of water. If a filter paper, holding a wet precipitate, be placed upon a layer of this earth, it will become quite dry in a very short space of time. Mr. Austen also remarked that substances retain their heat for several days when placed in cork boxes. To keep a substance air-tight, it may be placed in a flask, the neck painted with a solution of india rubber in chloroform, and a plate of glass laid upon it. The solvent quickly evaporates, leaving a delicate film of rubber, which holds the glass tightly in place.
The next meeting of the Chemical Section will be held February 12; of the Mineralogical Section, February 19.
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