VIETH METHOD. 157 



direct to the milk, and polarising the resulting filtrate. He 

 finds the volume of precipitated proteins from 100 c.c. of milk 

 to amount on the average to 3 c.c., and, consequently, adds 

 3 c.c. of acid mercuric nitrate solution to allow for this. The 

 method is carried out as follows : Measure 50 c.c. of milk into 

 a small flask, add 1-5 c.c. of acid mercuric nitrate, and mix well 

 by shaking violently ; pour the mixture on to a filter, and fill a 

 polarimeter tube with the filtrate ; polarise, and correct the 

 reading for that obtained in a blank reading i.e., by reading a 

 tube filled with water. 



As the [a] D of milk-sugar is 52-5, the reading, if in angular 

 degrees, can be converted into percentages of milk-sugar by the 

 following formula 



Where ra = number of grammes of milk-sugar per 100 c.c. of solution 



polarised. 



I = length of tube in millimetres. 

 r = reading in angular degrees. 



If a tube of 1984 millimetres be used (these tubes are sup 

 plied with the instrument used by Vieth), the formula becomes 



"1-042' 



If the length of the tube be 200 millimetres, the formula is 



r 



The resulting figure representing milk-sugar in the solution 

 polarised must be submitted to correction. 



The volume of the liquid from which the fat and protein have 

 been precipitated is the volume of the milk plus that of the 

 mercuric nitrate minus that of the protein precipitate and fat. 



As the volume of the mercuric nitrate was made purposely 

 equal to that of the protein, both of these may be neglected, one 

 compensating for the other. 



Taking the volume of the milk as 100 c.c., the volume of fat 

 in this will be the percentage by weight of fat multiplied by the 

 specific gravity of the milk, divided by the specific gravity of 

 the fat. 



The milk-sugar may be calculated either as hydrated or as 

 anhydrous sugar, but it is usual to calculate it in milk analysis 

 as anhydrous sugar. 



The following formula expresses the percentage of anhydrous 

 milk-sugar in the milk when a tube of 1984 mm. is used : 



