96 ELEMENTARY GENERAL SCIENCE CHAP. 



the masses of this volume of the kinds of matter we experiment 

 upon are a direct measure of the densities of these bodies. We 

 can thus define it : Density is the mass of a unit volume of 

 a substance. It follows from this definition that if the volume 

 of a body is multiplied by its density, we shall obtain its mass 



volume x density = mass 



or density = -- 

 volume. 



In using this relation between the volume and mass care must 

 be taken to use the proper units. In all scientific work it is 

 customary to adopt the cubic centimetre and gram as the units 

 of volume and mass respectively. 



Density can be regarded in another way. It is clear that if 

 we pack twice the amount of mass into a given volume we shall 

 have doubled its density, so that density may be looked upon 

 as the closeness with which a mass is packed into a given 

 volume. 



When we use the cubic centimetre and grain as the units of 

 volume and mass and apply the equation given above, the 

 density of water at 4 works out to be 1 for all other forms 

 of matter the number will be either a fraction or multiple of 

 this value. 



Relative Density. Hence the ratio of the weight of any 

 volume of a substance to the weight of the same volume of water 

 at 4 C. is equal to the absolute density of the substance, while, 

 since the density of water only changes slightly with the tempera- 

 ture, the ratio of the weight of the substance to the weight 

 of water at temperatures other than 4 C is a number very nearly 

 equal to its absolute density. This ratio is the relative density 

 of the body, or, as it is frequently called, the specific gravity. l 



When relative densities are used, care must be taken to indi- 

 cate the temperature of the water to which its weight is referred. 

 Thus, relative density alcohol 15C / 15 indicates the ratio of the 

 weight of equal volumes of alcohol and water, both at 15; 

 while if we wrote 15 7 4 it would indicate the ratio of the 

 weight of any volume of alcohol at 15^ to that of the same volume 

 of water at 4. 



The student should carefully notice the difference between the 



l The term specific gravity is so gun orally used in this sense that this meaning 

 is here given. It is, however, preferable to restrict the term to its true meaning, 

 i.e., the -icciy1it of unit volume, and to always use the term relative density when 

 the above ratio is meant. 



