9

magnitudes of the two ingredients. This, however, is notuniversally the case; an increase or diminution of magnitudeoften attending the combination of two different ingredients:thus a cubical inch of alcohol, combined with a cubical inchof water, forms a compound which will measure less than twocubical inches : and a cubical inch of tin in a fluid state, mixedwith one of lead, forms a compound exceeding two cubicalinches in magnitude. But this consideration belongs ratherto chemistry.

22. That the specific gravities of bodies may be morereadily compared, it is expedient to assume the specific gra-vity of some substance as a standard. That of any fluid orsolid might be made use of, for this purpose : but in orderthat the comparison should be made under the same circum-stances, it is necessary that the substance should be always thesame in respect of its nature and density. Distilled water' has been generally taken, at a temperature of 39° Fahrenheit,(a certain temperature being necessary, since heat causes a vari-ation in magnitude); or where this cannot readily be procured,rain water, which has sensibly the same degree of purity,and of which we have every reason to believe the density, whenat the same temperature, invariable. It is also particularlyconvenient for the purpose, as a cubic foot of it weighs 1000ounces avoirdupois.

Let the specific gravity therefore of water be consideredas a standard, and represented by 1000; to determine, on thesame scale, the specific gravity S of any other body, whose,weight in avoirdupois ounces is W , and magnitude in cubicfeet M.

Since (II), Woe MS, ioOO : W :: 1 X 1000 : MS;

.-. = MS,

from which equation the specific gravity may be found.

If, as is the case in tables of specific gravities, that ofwater be assumed = 1, those of other bodies will be determinedby moving the decimal point three places farther to the left-

B