SOURING AND DECAY 131 



spoiling decay. If the bacteria had not been present 

 there would have been no change at all .in the food. So 

 the bacteria spoil our food and thus are our enemies. If 

 the only effect which the bacteria have upon food were 

 merely to spoil it we would not have to take so much 

 care of it. In addition to spoiling the flavor of our food 

 and causing it to have a bad odor, the bacteria produce 

 certain poisons, called ptomaines. The ptomaines may 

 be formed to a degree sufficient to poison us without 

 there being any disagreeable odor or flavor to the food. 

 These poisons are very dangerous and we should be 

 careful not to eat old food, or food which has remained 

 long in a warm place. Cooking the food does not destroy 

 the ptomaines. 



Another kind of bacteria changes cider and grape 

 juice into vinegar. The mass of slimy material which is 

 always found in vinegar, called mother of vinegar, is com- 

 posed entirely of countless millions of bacteria. Vine- 

 gar is one of the class of materials which are called acids. 

 You can always detect an acid by its sour taste. When 

 milk sours it is because an acid has been produced in it 

 by bacteria. In the following experiment on milk we 

 can learn what is necessary to preserve food of all kinds. 



Experiment 63. How to Preserve Milk. 



Apparatus: Can or dish in which to boil water, 

 ring stand, burner, 4 test tubes or small bottles. 



Materials: Milk, borax, cotton, labels, boiling soda. 



a. Label the test tubes No. 1, 2, 3, 4 and fill them 

 one-half full of fresh milk. Set one aside, just as it is, 

 in a warm place, put the second one in as cold a place as 

 possible. To the third add a pinch of borax and set in 



