VAEIATIONS OF SPECIFIC GRAVITY IN MILK. 63 



of the liquid. As milk has always the same composition within 

 narrow limits, there is no practical difierence in the height to 

 which it is attracted round the stem ; the eye soon becomes 

 trained in making the proper allowance for this. 



(3) Lactometers are only correct at the temperature at which 

 they are graduated ; at other temperatures their volume varies ; 

 no inconvenience on this account is felt in practice, as this is 

 allowed for in the tables given for correcting the specific gravity 

 to a temperature of 15-55° C. (60° F.). 



Practical instructions for the use of lactometers will be j;iven 

 later under the " Testing of Milk." 



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 r030 and 1-034. The average specific gravity of milk is r03-2'2. 



The specific gravity is dependent on two causes — the amount 

 of solids not fat, which, being dissolved in water, raises 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 arc from r0135 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 r030 and 

 r034. 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 suflScient cream be 

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

 gravity will be found to be about r028. 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. 



As stated above, the specific gravity is raised by the solids 



