The Relation between Weight and Volume 65 



35. In practice it is usual to compare the densities 

 of all substances with that of a standard substance, 

 the standard in general use being the density of pure 

 water. In the last chapter we learnt that the weight 

 of 1 c.c. of pure water was 1 gram ; hence the density 

 of water is 1 g. per c.c. The densities of lead, iron, 

 and wood, as given above, are respectively 11*2, 7*8, 

 and O7 times the density of water. To the numbers 

 11*2, 7' 8, and 0'7, when used in this connection, a 

 special term is applied. They are called the " specific 

 gravities " of lead, iron, and wood. This name is not 

 happily chosen. It may be paraphrased as " special 

 weights". A specific gravity is not, however, a weight 

 but simply a number, and a much more apt designation 

 for it is "relative density". Thus we may define the 

 specific gravity or relative density of a substance as "the 

 number of times that the density of the substance is 

 as great as the density of pure water ", or as " the ratio 

 between the density of the substance and that of pure 

 water". 



36. The specific gravity of a substance is some- 

 times confused with its density. This confusion should 

 not arise, however, if it be remembered that the specific 

 gravity is merely a number, being the value of a ratio, 

 e.g. the specific gravity of iron is 7'8 ; while the density 

 is a measure of the mass of unit volume of the substance 

 and requires for its complete statement that mention 

 be made of the names of both the unit of mass and of 

 volume involved, e.g. the density of iron is 7*8 g. 

 per c.c., or 4'51 oz. per cu. in. 



Exercise for Student. 



Why is the number of grams per c.c. in the statement of the 

 density of a substance the same as the number which denotes its 

 specific gravity ? 



H. D. s. 5 



