350 



THE SMALLEST PLANTS (BACTERIA! 



found free from disease are allowed to sell their milk as 

 "certified," though the meaning of this term varies. Not 

 only is the raw milk from tubercular cows dangerous, but 

 also the butter and cheese made from it. 



Bacteria multiply rapidly and remain active while milk 

 is warm, and so it should be cooled as soon as possible 

 after it has been taken from the cow. Milk should not 

 be used when it is too old, for in that case the harmless 



bacteria may all have died and 

 harmful ones taken their places. 

 Milk should not be left in a 

 metal container, nor open to 

 the air, nor placed in an ice 

 chamber where it can absorb 

 the odors of other foods. 



Ice cream should be eaten 

 only when fresh, for poisons 

 (ptomaines) are formed by the 

 action of bacteria, especially in 

 ice cream which has been melted 

 and then refrozen. Ice cream 

 should be made under clean and healthful conditions, and 

 should never be exposed to the air of the street. 



Men ivho made the Study of Bacteria Possible. — The 

 inventor of the microscope should be placed at the head 

 of the list of men who made the study of bacteria possi- 

 ble, for without this instrument we should not know that 

 such plants exist. We do not know who the actual in- 

 ventor was, but the microscope was little more than a 

 toy until it was improved by a Dutch naturalist, Leeu- 

 wenhoek (Lu'wen-hook) in the latter part of the seven- 

 teenth century. Next in the study of bacteria comes 

 Pasteur, who discovered and studied them in their rela- 

 tion to the souring of milk and in other fermentations. 



Figure 371. — Beef Jelly. 

 Exposed in sanitary dairy. 



