2 THE GENERAL CHARACTERS OF MICROORGANISMS 
proved yeast to be a living plant. Leeuwenhoek was also first to 
see bacteria, and studied them as early as 1695. His descriptions, 
considering the fact that he had only simple lenses to work with, 
were remarkably correct. Even his suggestions concerning their 
nature sound quite modern and were certainly superior to much of 
the speculation that followed. He intimated that they mightbe 
the cause of disease. But for 15 years after Leeuwenho k, al- 
though the microscope became a familiar plaything, it was hardly 
thought that these minute organisms offered a subject for serious 
study. Foracentury they were simply objects of speculation, and 
many were the exclamations which they excited as to the wonders 
of nature, with here and there a suggestion as to their possible im- 
portance in producing certain natural phenomena. 
Relation to Disease.—Not until toward the middle of the 
nineteenth century was it conceived that the microscopic organ- 
isms, at first grouped together under the general head of animal- 
cule, could have more than scientific import. At that time there 
began to appear suggestions as to their possible relations to certain 
diseases, and almost simultaneously they were thought of as caus- 
ing fermentaitons. Even before it was known what yeast was, it 
was recognized as in some way associated with alcoholic fermenta- 
tion; but not till about 1838 was it clearly proved that yeast plants 
are the cause of the fermentation of sugar. The development of a 
knowledge of bacteria followed a little later. One of the first real 
contributions to a knowledge of their significance was the demon- 
stration in 1840, of the fact that certain microscopic organisms 
cause blue milk. Thus, at the very beginning of the modern study 
of bacteria, they were associated with peculiar agricultural phenom- 
ena, an interesting fact when we notice that, in the next quarter 
of a century or more, the chief investigations, and all the interest 
in them, centered around the question of their agency in producing 
disease. Bacteria are still suffering in reputation from the fact 
that, for thirty years, they were studied by microscopists chiefly 
from the standpoint of their agency in the production of disease. 
