Text-book of chemistry : inorganic and organic with toxicology for students of medicine, pharmacy, dentistry and biology / by R.A. Witthaus.

  • Rudolph August Witthaus
Date:
1919
Catalogue details

Licence: Public Domain Mark

Credit: Text-book of chemistry : inorganic and organic with toxicology for students of medicine, pharmacy, dentistry and biology / by R.A. Witthaus. Source: Wellcome Collection.

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    unit volume of that substance. But as tlie weight of a given volume of a substance, particularly in the liquid or aeriform state, varies with differences of temperature and of pressure, a definite tempera¬ ture and pressure have been arbitrarily selected as constituting normal conditions. The temperature is 0°C., and the pressure that of a column of mercury 76 centimeters high at 45° latitude. Pressures are measured either by the height of a column of mer¬ cury which the pressure will sustain in opposition to gravity, in cm. or mm.; or in atmospheres, one atm. being the pressure which will sustain a column of mercury of the average height of the barometer; i. e., 760 mm. As the specific gravity (below) of mercury is 13.6 at 0°C., 1 cc. of mercury weighs 13.6 gms., and each mm. of mercurial column is equivalent to a pressure of, 1.36. per sq. cm., and one atm. of pressure is equal to 1033.6 gms. per sq. cm. The specific volume of a substance (Vs) is the reciprocal of its absolute density: Vs=i, and is the volume, in cc., which one gram occupies under normal conditions. Thus for hydrogen: .ooirog =11111 cc., or 11.11 L., for oxygen: .-totVsf =699.7 cc., and for air: .FTrnnFTi —^ /3.4 cc. States of Matter.—Matter exists in the three forms of solid, liquid and gas (or vapor). The term fluid applies to both liquids and gases; the former being distinguished as incompressible, the latter as compressible fluids. Cohesion is the force by which molecules of the same kind are held together. It is most active in solids, which therefore have definite shape and magnitude. In liquids it is much less active, yet sufficient to maintain a definite magnitude of the liquid, but it is in part overcome by gravity, which causes the liquid to assume the shape of the containing vessel. In gases cohesion is almost nil; therefore, the shape and volume of any gas are those of the containing vessel. Cohesion diminishes with the addition of heat; therefore, by adding heat to a solid it is, if not decomposed, converted into a liquid and then into a gas. SPECIAL PROPERTIES OF SOLIDS, LIQUIDS AND GASES. Crystallization.—Solid substances exist in two forms, amor¬ phous and crystalline. Amorphous substances assume no geometric shape; they conduct heat equally well in all directions; they break irregularly; and, if transparent, allow light to pass through them equally well in all directions. A solid in the crystalline form has a definite geometrical shape; conducts heat more readily in some directions than in others; when broken, separates in certain direc¬ tions, called planes of cleavage, more readily than in others; and modifies the course of luminous rays passing through it differently when they pass in certain directions than when they pass in others.