34 HUMAN PHYSIOLOGY CHAP. 



cheese, or a bunch of onions, and milk are placed side 

 by side, the latter will soon be tainted with the odor of 

 the former. Vessels containing milk should always be 

 covered, or, still better, sealed. 



Milk of diseased cows or that which has not been 

 kept clean is dangerous to use. Typhoid fever and 

 other serious diseases are often due to the use of im- 

 pure milk. We should use fresh, wholesome milk more 

 and more with our meals instead of tea and coffee and 

 other less valuable beverages, for it is an excellent 

 drink and at the same time a very nutritious food. 



Place some milk in a shallow pan, in an ice box, side by side with 

 onions, cheese, or vegetables, and notice that the milk becomes 

 tainted in a short time. 



37. Alcohol. If sweet apple juice be allowed to 

 stand for some time in a warm place, it slowly loses its 

 sweetness and finally becomes strong and biting to the 

 taste. What has happened ? The sugar of the apple 

 juice has been changed into a gas, called carbon dioxid, 

 that you can see bubbling up 

 through the liquid and escaping 

 into the air, and into alcohol 

 which remains in the liquid and 

 gives it its strong biting taste. 



What makes the sugar break 

 up into carbon dioxid and alco- 



FIG. 12.- Yeast plants hol p There j s a tiny yeast p l ant 



(Buchholz). 



on all fruits and in the air. When 



the apples were crushed this plant was in the liquid and 

 began to live on the sugar as its food. The yeast plant 



