76 PHYSICAL DETERMINATIONS. 



proceeds alone. Only in those cases where the sample is so 

 small that the lactometer will not float conveniently, if the 

 quantity necessary for total solid estimation has been removed, 

 it is usually convenient to take the specific gravity first. 



Variation in Milk. The specific gravity at 15-55 C. (60 F.) 

 of the milk of individual cows varies from 1 -0135 to 1*0397 ; when 

 the mixed milk of a herd is tested it rarely falls outside the limits 

 of 1 -030 and 1 -034. The average specific gravity of milk is 1 -0320. 



The specific gravity is dependent on two causes the amount 

 of solids not fat, which, being dissolved in water, raise the 

 specific gravity ; and the fat, which, being lighter than water, 

 lowers it. By removing the fat (with a small proportion of 

 other constituents) as cream the specific gravity of the milk is 

 raised. By the addition of water the specific gravity is lowered. 

 The specific gravity has been and is largely used as a test for 

 the addition of water to milk ; for the detection of large amounts 

 of water to milk it has some value. 



That it is a test of the roughest kind is shown by the following 

 facts : 



(1) The variations in specific gravity are from 1-0135 to 

 1-0397 i.e., nearly twice its bulk of water could be added to 

 milk of the highest specific gravity to reduce it to the lowest. 

 These, of course, are exceptional cases, and the specific gravity 

 of the mixed milk of a herd is nearly always between 1 -030 and 

 1-034. At least 10 per cent, of water could be added to milk of 

 1-034 specific gravity before it would be suspected by this test. 



(2) A milk of 1 -032 specific gravity, if the cream is all removed, 

 would give a product of about 1-036 specific gravity; and an 

 addition of rather more than 10 per cent, of water would bring 

 the specific gravity back to 1-032. 



(3) If to milk of 1 -032 specific gravity sufficient cream be added 

 to raise the percentage of fat 4 per cent., the specific gravity will 

 be found to be about 1-028. The same result would be arrived at 

 were the milk allowed to stand, and the upper portion removed. 



As an absolute test the specific gravity is liable to be greatly 

 misleading ; as a preliminary test it is of the greatest import- 

 ance, and should never be neglected. 



Specific Volume. By our definition of specific gravity, 



W 1 V 



we write o = ^ ; we may also write, ^ = f \ or, in words, 



= expresses the volume of 1 gramme ; this is called specific 



a 



volume. The expression =r is, therefore, a mode of indicating 



specific volumes ; as G (degrees of specific gravity) is 1,000 times 



