352 THE ANALYSIS OF MILK AND MILK PRODUCTS. XV. 



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 pellicle, is tough and impervious and 

 interferes with the evaporation 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 re- 

 peatedly, 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 usually per- 

 formed by means of a modified hydrometer known as a 

 "lactometer," the graduations usually ranging from to 40, 

 the reading of the instrument, sometimes known as "lacto- 

 meter degrees," really giving the amount by which the density 

 of the milk exceeds 1000 when the density of water is taken as 

 1000. Thus on the lactotneter 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 and 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 



