Sect 2. Pharmaceutical Operations.

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be ascertained by comparing the weight of a solid body, suchas a piece of crystal, when immersed in distilled water, withits weight when immersed in the fluid we wish to examine;by dividing its loss of weight in the fluid by its loss of weightin the water, the quotient is the specific gravity of the fluid :or a small phial, containing a known weight of distilled wa-ter, maybe filled with the fluid to be examined, and weighed,and by dividing the weight of the fluid by the weight of thewater, the specific gravity is ascertained.

Although these are the onlygeneral principles by whichspecific gravities are ascertained", yet as the result is alwaysinfluenced by the state of the thermometer and barometer atthe time of the experiments, and as the manipulation is a■work of great nicety, various ingenious instruments have beencontrived to render the process and calculation easy. Of allthese, the gravimeter of Morveau seems to deserve the prefe-rence.

It would be of material consequence to science and the arts,if specific gravities were always indicated by the numericalterm expressing their relation to the specific gravity of distill-ed water. This, however, is unfortunately not the case. Theexcise in this country collect the duties paid by spiritous li-quors, by estimating the proportion which they contain of astandard spirit, about 0.933 in specific gravity, which theycall hydrometer proof; and they express the relation whichspirits of a different strength have to the standard spirit, bysaying that they are above or under hydrometer proof. Thus,one to six, or one in seven below hydrometer proof, means,that it is equal in strength to a mixture of six parts of proofspirit with one of water.

The only oilier mode of expressing specific gravities, whichit is necessary to notice, is that of Baume's areometer, as it isoften used in the writings of the French chemists, and is littleunderstood in this country. For substances heavier than wa-ter, he assumes the specific gravity of distilled water as zero,and graduates the stem of his instrument downwards, eachdegree being supposed by him to express the number of partsof muriate of soda contained in a given solution ; which, how-ever, is not at all the case. For substances lighter than wa-ter the tube is graduated upwards, and this zero is afford-ed by a solution of 1 of salt in 9 water. In the appendix,tables are given of the specific gravities, corresponding withall the degrees of both of these areometers, from Nicolson'sJournal.

The specific gravity of the gases differs so much from that