PLANTS WHICH CAUSE DECAY 377 



disagreeable, it is preferable to pasteurize 1 it, i. e., to 

 place the bottle of milk in water which is heated to 155 

 F. for twenty minutes, and then cooled. In practice it 

 is usually more convenient to heat the water to 155 F. 

 and then set it aside to cool slowly. Treat several 

 bottles of milk in this way (they should be stoppered 

 with cotton, or well corked). How long does it keep 

 sweet (a bottle once opened must be discarded) ; do 

 you detect any unpleasant flavor due to the heating? 



Pasteurizing is a legitimate process, but the use of 

 formalin and other preservatives by milkmen is to be 

 strongly condemned ; milk so treated is especially 

 harmful to infants. A rough test for formalin which is 

 sometimes used is as follows : Pour ordinary com- 

 mercial (impure) sulphuric acid slowly into a glass of 

 milk, letting it run down the side of the glass. If a 

 purple color appears at the junction of the milk and 

 acid it indicates the presence of formalin, and the 

 milk should be taken to the health officer for a test. 

 Put a little formalin in some pure milk and make the 

 test : how great a dilution will give the test ? 



We notice that the sulphuric acid curdles the milk : 

 this is due to the fact that the milk contains a proteid 

 called casein, which (like the white of an egg) is 

 coagulated by acids (cheese is made from the casein 

 of the milk). The curdling of milk is caused by an 



i See an article in the Year-Book of the U. S. Dept. of Agriculture for 1894 

 by Schweinitz ; for 1895 by Moor*. 



