CALCULATION OF TOTAL SOLIDS. 353 



fat of a sample of cows' milk are related to each other in such 

 a manner that it is possible to calculate the value of any one 

 of these three quantities if the other two are known. This can 

 be done because an increase in the solids not fat produces a 

 rise in the specific gravity, while the more fat there is present 

 the lower will be the specific gravity. Many formulae have 

 been devised to facilitate the calculation. One of the most 

 convenient is that of Eichmond : 



T = 1-2F + 0-25G + 0-14, 

 where 



T = percentage of total solids. 

 F = percentage of fat. 



G = " lactometer degrees" (i.e., sp. gr. x 1000 1000). 

 The results obtained for total solids from the fat and specific 

 gravity by this formula agree closely with actual determina- 

 tions. 



Determination of Proteids. The total amount of albu- 

 minoids in milk can most easily be deduced from the amount 

 of total nitrogen. This is conveniently done by the Kjeldahl 

 method described on p. 94. From 5 to 10 grammes of milk 

 are taken and 20 or 25 c.c. pure sulphuric acid, the rest of the 

 process being performed as already described in the case of 

 soils. By multiplying the total nitrogen by 6'38 the total 

 proteids are obtained, since both casein and albumin contain 

 15-7% of nitrogen. 



If separate determinations of the amounts of casein and 

 albumin be required, the casein must be precipitated. This 

 can be effected by the addition to the milk of twice its volume 

 of saturated magnesium sulphate solution and of the powdered 

 salt until saturation is complete. The casein can then be 

 filtered off, washed with saturated magnesium sulphate solu- 

 tion, and the nitrogen in it determined by the Kjeldahl process. 

 The percentage of nitrogen found multiplied by 6*38 gives the 

 percentage of casein. The albumin can then be found by 

 difference, for the amount of the other proteid of milk, the 

 globulin, is so small that it may be neglected. * 



* Sebelien, Zeits. fiir Physiol. Chemie. 13, 137 and H50. 



AA 



