90 A.VALySIS OF MILK. 



excess of mercury present does not necessitate any appreciable 

 variation of the factor. 



Estimation of Milk-Sugar. — Mill^-sugar is generally estimated 

 indirectly, as it is not possible to isolate it quantitatively from 

 milk in a state of purity. The following metliod may, however, 

 be used to obtain an approximate determination of the milk- 

 sugar : — 



By Alcohol. — To 10 c.c. of milk add 20 c.c. of 90 per cent, 

 alcohol, well mix and filter ; of the filtrate take 10 or 15 c.c, 

 evaporate to dryness on a water-bath and dry at 100° C. (212" F.) 

 till the weight is constant. Ignite the residue and weigh the 

 ash. The weight of the residue less the weight of the ash will 

 give the weight of the milk-sugar. The volume of the aqueous 

 portion must be calculated ; on mixing alcohol and water a 

 contraction takes place ; this with the c^uantities given is 0'4 c.c. ; 

 the volume occupied by the protein is on the average 0'25 c.c. ; 

 the volume of the fat is obtained by multiplying the percentage 

 by weight by O'lll. 



The percentage of milk-sugar is obtained by the following 

 formula : — 



,1 = 30 -(0-05 + 0-111 F) ^ ^ ^^ 1 



where 



X = mimlior of c.c. taken for estimation of residue, 

 D = spec-itic graA-itj- of milk. 

 F = percentage of fat in milk. 

 R = weight of residue. 

 A = ,, asli. 



M = percentage of milk-sugar. 



This method has a tendency to yield results about 0-2 to 0-3 per 

 cent, too high. 



By the Polariscope (Fig. 6). — The quickest method of 

 milk-sugar estimation is bj' the polariscope ; before the milk 

 can be polarised it is necessary to remove the fat and proteins 

 completely which interfere either by making the solution too 

 opaque for reading or by polarising to the left. 



Wiley's Method.— The investigations of AViley have shown 

 that mercury compounds are the most efficient for this purpose, 

 of which " acid mercuric nitrate " is the most convenient. This 

 is prepared as follows : — Mercury is dissolved in twice its weight 

 of nitric acid of specific gravity i-42, and, after solution, an equal 

 bulk of water is added. 



Basic lead acetate has also been used to remove fat and protein, 

 but Wiley has proved that the results are not accurate, owing to 

 the incomplete removal of protein. Still more inaccurate is the 

 use of acetic acid, followed by boiling, which has been recom- 

 mended by Blyth. 



