316 
SMALLEST PLANTS ( BACTERIA) 
tubercular cows dangerous, but also the butter and cheese 
made from it. 
Bacteria multiply rapidly and remain active while milk 
is warm, consequently 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, or 
open to the air, or 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. 
271. Men Who 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 possible, 
for without this instrument we should not know that such 
plants exist. We do not know who the actual inventor 
was, but the microscope was little more than a toy until 
it was improved by a Dutch naturalist, Leeuwenhoek 
(Lu'wen-hook), in the latter part of the seventeenth century. 
Next in the study of bacteria comes Pasteur, who discovered 
and studied them in their relation to the souring of milk 
and in other fermentations. Next comes Koch, who dis¬ 
covered a way of separating bacteria so that each kind may be 
studied by itself, a method called getting a “ pure culture.” 
He also invented the tuberculin test. Most of our facts 
about bacteria have been learned during the past thirty-five 
years, since men have learned how to prepare them for study. 
272. Healthy Bodies and Bacteria. — So much has been 
said about harmful bacteria that a word of caution is needed. 
Two facts should make us take a sane view of the situation: 
(1) for every harmful bacterium there are thousands of 
