MAXIMUM DENSITY OF WATER. 



779 



standard of comparison for other bodies that is, it is 

 assumed as = 1, and the number which expresses ex- 

 actly the specific gravity of any other body indicates 

 how many times heavier that body is than an equal 

 volume of water at the temperature of 4. The gramme 

 is the weight of one cubic centimetre of water at 4. 



The temperature of the body which is compared with 

 water is taken as 0, unless stated otherwise. For 

 example, when the specific gravity of mercury is said 

 to be 13-596, this means that any quantity of mercury 

 is 13*596 times as heavy as a volume 

 of water at 4, equal to the volume which 

 the mercury occupies at 0. When no 

 great exactness is required the specific 

 gravity of water may also be assumed 

 as = 1 at temperatures which do not con- 

 siderably differ from 4, or rather the 

 small error in such cases may be neg- 

 lected. The exact specific gravity of 

 water is, however, at 0, O99D88; at 10, 

 0-99975; at 20, 0-99831; but at 100 it 

 is sensibly less, viz. 0*9588. 



The determination of the coefficient of 

 expansion of liquids is a somewhat 

 laborious operation, because liquids must 

 be contained in vessels of some kind, 

 and changes of temperature affect not 

 only the liquid but at the same time also FIG. 388 ( real size}. 

 the vessel. The change of volume which water under- 

 goes near its freezing-point is especially difficult to 

 determine with accuracy, because the magnitudes to 

 be measured are exceedingly small ; thus, the expansion 



