MODES OF EXPRESSION. 67 



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. 



This formula is strictly true only if the weights are taken in a 

 vacuum. 



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, 

 both in air. All specific gravities in this volume are stated in 

 this way unless otherwise mentioned. In order to avoid con* 



fusion the symbol specific gravity at is often used to 



10*00 



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 JQ or -JQ- is 



used to express the true specific gravities at 15*55 or 20. 



Occasionally it is convenient to compare the weight of a liquid 

 at some other temperature with the weight of water at that 

 temperature ; thus the specific gravity of fats taken may be 



100 

 at 100 C., and the expression specific gravity at 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 



20 

 termed the " apparent specific gravity in glass at " (or 



whatever the temperature may be). 



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



determined as " apparent specific gravities in glass at 



