LAW OF MIXTURES. 



85 



in 100 c.c. of the mixture, AS is the number of grammes of this 

 substance in 100 c.c. 



From the equations above given we can deduce the law that 

 " if two substances of differing specific gravity be mixed, the 

 specific gravity of the mixture will be equal to the specific gravity 

 of one of the substances plus the number of grammes of the other 

 per 100 c.c. of the mixture multiplied by a constant factor." 



Regarding milk as a mixture of fat and a solution of solids 

 not fat in water, we can say that the specific gravity of a milk 

 is equal to the specific gravity of the solution of solids not fat plus 

 the number of grammes of fat per 100 c.c. multiplied by a constant. 



In the solution of solids not fat we have in 100 c.c. of it x 

 grammes of solids not fat ; let us assume that their density is y. 



if 



Then x grammes will occupy a volume -. Let the specific gravity 



u 



of the solution be S. The 100 c.c. weigh 100 S grammes, and 

 the water in this weighs 100 S x grammes ; it also measures 



100 - c.c. 



y 



Now, as the specific gravity of water is 1, 



100 S - x = 100 - 

 V 



100 S = 100 -f x - * 



(3) 



Now, ~r is a constant, provided that y remains constant, 

 luu y 



Putting the equation into words we find " that the specific gravity 

 of a solution of solids not fat is equal to 1 plus the number of 

 grammes of solids not fat in 100 c.c. multiplied by a constant." 

 It is known, however, that the specific gravity of substances in 

 solution is not quite constant, but varies slightly with dilution. 



The following figures (Table XVII.) will show that in milk 

 the law just enumerated holds good within the limits of experi- 

 mental error. A poor skim milk was diluted with water, and the 

 total solids and specific gravity at 15-55 estimated : 



TABLE XVII. 



