i MILK 89 



sulphuric acid and 7'5 c.c. of mercuric iodide solution are added, 

 the liquid made up to 100 c.c., and the filtrate polarised. If 

 now the rotation found with 75 c.c. of milk alone is deducted 

 from that just obtained, then the apparent amount of added 

 milk sugar in grams gives M 2 of the formula. 



(c) The volume of the precipitate is calculated in the fol- 

 lowing manner : the volume of the liquid and that of the pre- 

 cipitate (NV) = 100 c.c. ; this volume corresponds to M 2 ; the 

 volume of the liquid alone =100 NV\ this corresponds to 

 M^ or 



100: 



(d) The actual amount of milk sugar in the milk is obtained 



, , 100 -NV 

 by multiplying the apparent amount by -- . 



3. The Refractometer Method. 



When Wollny had worked out his method for the estimation 

 of fat in milk by the refractometer, he also proposed to use the 

 refractometer for determining the milk sugar. Wollny also 

 prepared a table for this purpose, but the method was allowed 

 to lie unnoticed until R. Braun 1 took it up again in 1901. 

 Braun found that this method, which is the simplest of all, 

 gives results which agree very closely with those obtained by 

 gravimetric analysis. The author of this book can corroborate 

 Braun's conclusions from his own experience. 



The way in which the method is carried out is as follows : 

 5 c.c. of milk are put into the ordinary Wollny test-glass (see 

 Fig. 15, p. 45), and 5 drops of 4 per cent, calcium chloride 

 added. The test-glass is then closed with a cork, wrapped in a 

 cloth for fear of accident, and heated for 10 minutes in a bath of 

 boiling water. The tube is then cooled by being placed in cold 

 water, and then brought to a temperature of 17 '5 C. A drop 

 of the serum which has separated out, and must be quite clear, 

 is brought between the prisms of the refractometer, and the 

 percentage amount of milk sugar can be read off with the help 



1 Mikh-Zeitung, 1901, p. 578. 



