XVI. THE ANALYSIS OF MILK AND MILK PRODUCTS 351 



dried and examined ; it was found to be calcium sulphate. Its forma- 

 tion furnishes a somewhat striking proof of the large amount of calcium 

 compounds present in cows' milk. 



These rapid centrifugal methods have repeatedly been 

 compared with the gravimetric processes for fat deter- 

 minations, and the results have always proved to be in 

 close agreement, the error being rarely more than Ol per 

 cent. 



Determination of Total Solids. The principle of 

 all the methods is simply to expel water, by heat, from a 

 known amount of milk and determine the weight of the 

 residue. Direct evaporation of milk in a dish is slow and 

 difficult, owing to the formation of a skin, consisting 

 mainly of proteids, upon the surface ; this skin or pel- 

 licle is tough and impervious and interferes with the eva- 

 poration of the liquid below it. Many methods for avoiding 

 the formation of the pellicle have been devised. One of 

 the best is the following : 



A platinum dish containing about 10 grammes of 

 recently ignited sand and a short piece of glass rod is 

 weighed ; 10 c.c. of milk are then run into it from a 

 pipette. The dish is then placed on a water bath and 

 the sand and milk stirred repeatedly, at intervals, until 

 the mixture is apparently dry. Two hours further heating 

 in a water bath or, better, in an air bath at 105-110 is 

 then generally sufficient to drive off all moisture. The 

 dish is then cooled in a desiccator and weighed. 



Determination of Specific Gravity. This is usu- 

 ally performed by means of a modified hydrometer known 

 as a " lactometer," the graduations usually ranging from 



to 40, the reading of the instrument, sometimes known FIG. 16. Ger- 



as "lactometer degrees," really giving the amount by ber tube, 

 which the density of the milk exceeds 1000 when the One-half na- 

 density of water is taken as 1000. Thus on the lact- 

 ometer scale would be the point to which the stem sinks in pure water, 

 while 40 would be the point to which it sinks in a liquid whose specific 

 gravity is 1-040 (water = 1) or 1040 (water = 1000). The lactometer, 

 though easy and convenient to use, is not capable of great accuracy. 



A specific-gravity bottle holding 25 or 50 c.c. affords much greater 

 accuracy, though a determination requires more time. It will be found 

 most convenient to determine the weight of the empty bottle and of the 

 bottle filled with distilled water at a temperature slightly higher than 

 the average temperature of the room arid to use these values always. 

 In any particular case, then, only one weighing that of the bottle 

 filled with milk at the temperature used before is necessary. A 

 more rapid and very accurate method of determining the specific gravity 

 of milk is by means of the " Westphal balance ". 



It has been found that the specific gravity, total solids and fat of 



