SPECIFIC GRAVITY. 59 



divided by the specific gravity at that temperature — ia the case 

 taken 0-99577. 



The following formula may be used for determining specific 

 gravity :— 



„ _ wt. of a known vol. of liquid x sp. gr. of water at same temp. 



" wt. of same volume of water at same temperature. 



In practice it is customary to assume that water at 15-55° C. 

 (60° F.) has the specific gravity 1. 



Thus, to ascertain the specific gravity at 15-55° C. (60° F.) it is 



customary to weigh a known volume of liquid, and to compare it 



with the weight of an equal volume of water at that temperature. 



All specific gravities in this volume are stated in this way unless 



otherwise mentioned. In order to avoid confusion the symbol 



15-55° 

 specific gravity at p.^.^ is often used to express this mode of 



expression. This means that the weight of a volume of liquid 

 at 15-55° is compared with the weight of an equal volume of 

 water at 15-55°. 



15-55' -20 . 

 Similarly, the expression specific gravity at ^^o - or .o is 



used to express the true specific gravities at 15-55 and -20" 

 respectively. 



It is occasionally convenient to compare the weiuht of a liquid 

 at some other temperature with the weicjlit of water at the same 

 temperature ; thus the specific gravity of fats is taken sometimes 



100' . 

 at 100° C, and the expression specific gravity at zr^ is used to 



express the value obtained by dividing the weight of a volume 

 of fat at 100° C. by the weight of an equal volume of water at 

 100° C. 



If we ascertain the weight of water held by a certain vessel at 

 a definite- temperature, we can ascertain the specific gravity of 

 any liquid by filling it with the liquid at the same temperature 

 and weighing it. If we fill the vessel with the Liquid at any 

 other temperature, the volume contained will not be the same 

 as that of the water, owing to the expansion of the vessel itself 

 altering the capacity. Nevertheless, specific gravity is frequently 

 ascertained on the assumption that the vessel does not alter in 

 capacity by change of temperature. As the vessel is usually 

 made of glass, this mode of expression of specific gravity may be 



termed the " apparent specific gravity in glass at -^^.^^o " (or 

 whatever the temperature may be). 



As a matter of fact, specific gravities of milk are usually 



