METHODS OF TESTING BUTTER FOR WATER 113 



that the water has been completely evaporated, the 

 dark color and pungent odor showing that other but- 

 ter constituents are beginning to undergo decomposi- 

 tion ; heating should then cease at once. So long as 

 any snow-white portions remain, the evaporation is 

 not complete. 



The escape of water-vapor is facilitated by shaking 

 the dish with a rotary motion from time to time during 

 the evaporation, thus breaking up the surface covering, 

 which tends to prevent the escape of water-vapor. 

 Experience enables one quickly to detect the point at 

 which heating should be stopped. 



Another method of telling when evaporation is com- 

 plete is to hold the face of a mirror over the heated 

 sample after foaming ceases ; the process is complete 

 when no moisture gathers on the mirror. Care must 

 be taken to keep the mirror from becoming too warm 

 to condense water-vapor. This may be done by using 

 two mirrors alternately, allowing one to cool while the 

 other is being used. 



In the case of samples heated in an oven, it is well 

 to make some experiments. Remove the evaporating 

 dish, when the darkening of the residue appears, cool 

 and weigh, after which replace the dish in the oven 

 and heat again for 10 or 15 minutes; if the water 

 was removed by the first heating, the sample will weigh 

 the same as before, while decrease in weight after the 

 second heating will show that the first heating was 

 insufficient to complete the evaporation. A few trials 

 of this kind will enable one to tell very closely how 

 long it will be necessary to heat for complete evapora- 



