Elements of chemistry : including the most recent discoveries and applications of the science to medicine and pharmacy, and to the arts / by Robert Kane.

  • Kane, Robert, 1809-1890.
Date:
1842
Catalogue details

Licence: Public Domain Mark

Credit: Elements of chemistry : including the most recent discoveries and applications of the science to medicine and pharmacy, and to the arts / by Robert Kane. Source: Wellcome Collection.

Provider: This material has been provided by the National Library of Medicine (U.S.), through the Medical Heritage Library. The original may be consulted at the National Library of Medicine (U.S.)

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    an elevation, it will be seen that in b c the surface a a of the fluid is convex, in d e it is plane, and in fg it is concave.] The particles of a body being held at certain distances from each other by the balance of their attraction and repulsion : if, by the ap- plication of an external force, as pressure, they be brought nearer, so as to occupy a smaller volume, the body is said to be compress- ible. If, when the external force is removed, the body, by the mutual repulsion of its particles, regain its original volume, it is said to be elastic; if, on the contrary, it remains as when compressed, it is called inelastic. In nature there are few bodies perfectly elastic, and none which can be said to be perfectly inelastic. In solid bodies, when pressure produces a change of volume, some traces of it are permanent; but in liquids and in gases, the restoration to the origi- nal bulk appears to be complete. The amount to which solid and liquid bodies may be compressed is very small, so much so that very delicate methods are necessary to determine it. Thus it requires a pressure of about 400 lbs. upon each square inch of the surface of water to diminish its volume by the ToVo part. In gases, however, the repulsive force acting without interference, and the particles being at much greater distances from one another than in the liquid or solid form, the amount of com- pressibility becomes very much increased, and the law by which it is regulated extremely simple, being, that the volume of any gas varies inversely as the pressure upon it ; that it is doubled if the pressure be diminished to one half, and reduced to one half if the pressure upon its surface be doubled. Thus, supposing a gas to measure 100 volumes under the pressure of 20 lbs., Then with pressures of 80 . 40 . 20 . 10 . 5 lbs. The volume becomes 25 . 50 . 100 . 200 . 400. The gases which are used in chemical operations are liable to constant changes of volume, from the alterations in the weight of the surrounding atmosphere, by which they are always pressed ; and hence, before we can tell how much of a gas we really have obtained by any process, it is necessary to ascertain the amount of atmo- spheric pressure, and to allow for it. The pressure which the air exercises is measured by the barometer, in which a column of quick- silver balances the pressure of the air, and varies in height according as this changes; the height of this mercurial column being accurately measured by a scale applied to the tube of the barometer. In these countries, the height of the barometric column fluctuates between 28 and 31 inches, but the average height of a year is about 29-8 inches. For simplicity, a number very near this, 30 inches, is taken as the standard pressure ; and whenever the specific gravity, or the volume of a gas is given, without particular remark, this standard height of the barometer is understood to be the pressure. If, therefore, we have a gas at a different pressure, it is usual, and often neces- sary, to reduce r.s volume to what it should have been under the standard pressure or, as it is generally termed, to correct for pressure: to do this, we use the rule given above for the change of volume with the pressure. Thus, if, in an analysis of morphia, we obtaiu 4-54 cubic inches of nitrogen gas when the barometer is at 28-5 inches, we say that, expressing the volume at 30 inches by V 28 ^i ' V : 98-5 : : 454 ; 30, or V=—x4 54=4 313.