WATER SUPPLY 261 



amount to nothing more than a sHght back pressure on the 

 engine. 



The hot water may be drawn oft* from this tank when- 

 ever desirable and cooled in the same manner as the 

 cream, that is, by running it over the cream cooler B. 

 From the cooler the water should be run into a tank in 

 which it can be cooled to the desired temperature by 

 means of ice water. The water as it leaves the cooler 

 will have a temperature of from 60 to 65 degrees, so that 

 only enough ice will be needed to reduce the temperature 

 about 10 degrees. 



Fig. 70 also illustrates the method of heating water 

 for the boiler and for general washing. 



Determining the Purity of Water. The author has 

 found that a good idea of the purity of well water may 

 be had by adding about ten c. c. of the water to one pint 

 of sterile milk and keeping the inoculated milk at a tem- 

 perature of about 85 degrees for 36 to 48 hours. If the 

 water contains many putrefactive organisms it will pro- 

 duce an odor in the i^iilk which is akin to that of rotten 

 eggs. A control sample of sterile milk should always be 

 carried as a check on the efficiency with which the milk 

 has been sterilized. Obviously pathogenic bacteria can 

 not be detected by this method. 



