ON BALANCES. 71 



is to be distinguished from its specific gravity, which shows 

 its weight in relation to its volume, also when referred to a 

 uniform standard. In shorter terms, the density of a body is 

 the quantity of matter in a unit of its volume ; the specific 

 gravity of a body is the weight of a unit of its volume. The 

 relation of the bulk or volume of a body to its weight is thus 

 expressed, both as to its density and its specific gravity, and 

 these terms are often used indiscriminately. But the former 

 term is more strictly applicable to solid bodies, and specific 

 gravity to liquids and gases. The densities of bodies are in 

 the direct ratio of their masses, and the inverse ratio of their 

 volumes. 



The unit of density which is adopted is the volume of an 

 equal bulk of distilled water at its maximum density, that is 

 to say at the temperature of about 4 C., or 39 d Fahr. The 

 numerical value of the density of a body is usually obtained 

 by hydrostatic weighing, or weighing it in distilled water and 

 comparing the weight of water displaced by it with its own 

 ascertained weight in a vacuum. When no temperature is 

 stated, it is generally understood that the numerical value 

 assigned is that at C., or 32 Fahr. 



The density of a body may also be determined from its 

 cubic measurement in relation to its weight as compared with 

 a weight of known density, where the form of the body 

 admits of the measurement being accurately made. But this 

 is seldom practicable. 



As to No. 4 of the data, it is evident that the density of 

 each weight varies according to its temperature and rate of 

 expansion, and that these influences must be allowed fur 

 in determining its mean volume, in order to ascertain the 

 volume of air displaced by it during the comparisons. 



36. The actual mode of computing the weight in grains of 

 air displaced by a standard weight is by adding the logarithms 

 of the following terms : 



(1.) Of the barometric pressure in millimetres, deducting the 

 pressure of vapour in the air, in millimetres of mercury. 



(2.) Of the ratio of density of the air at the observed Centi- 

 grade temperature to the maximum density of water. 



(3.) Of the ratio of the density of the weight at C. to its 

 density at the observed temperature and its ascertained rate 

 of expansion. 



(4.) Of the weight of the standard in grains. 



