ISABEL MULFORD. 223 
distinctly visible in the mass. Their chemical and 
physiological effects, too, are very apparent and were 
made use of in practical life long before the bacteria 
themselves had ever been thought of. Wine and vine- 
gar, and beer and bread were made, and the risks at- 
tending their manufacture were known, but until the 
time of Pasteur, no one dreamed of bringing in the 
microscope as an accessory. He taught his country- 
men to know with certainty when the ferments used 
were pure, and when they were invaded by diseased or- 
ganisms. 
Bacteria are developed in connection with organic mat- 
ter whose vital energy is weakened orimpaired. When 
exposed to the atmosphere the juice of fruits ferments. 
The clear liquid becomes cloudy or turbid, and deposits 
asediment. Bright spots of color appear upon solids. 
A minute quantity of these substances placed under 
the microscope reveals myriads of minute organisms, 
the active agents in these changes. 
When these and other fungous growths were seen de- 
veloping in lifeless matter without any apparent cause, 
the theory of spontaneous generation arose. Under 
certain favoring conditions, imperfectly understood, life 
was supposed to assert itself and arise, like Phenix, 
from the ashes. Many eminent scientists, from the time 
of Aristotle down, believed that the lowlier forms of life 
were able to spring spontaneously into existence. A 
few dared to dispute the theory, and some interesting 
-discoveries took place, but its complete refutation was 
reserved to men of our day. Germs of these organisms 
were shown to exist in the air. the earth, the water. 
Some of the most interesting experiments known to 
science, were made by Pasteur, Tyndall and others, all 
serving to show that organic matter, free from these 
germs and cut off from contact with air, will resist 
decay for an indefinite period, and, even after the lapse 
107 
