PRINCIPLES OF BACTERIOLOGY 
SECTION I 
GENERAL BACTERIOLOGY 
CHAPTER I 
HISTORICAL 
While the existence of minute organisms and their 
causal relation to some diseases may have been suspected 
long before the actual proof came—thus the conception 
of contagion, i. e., transmission of a disease from man 
to man, is mentioned by Aristotle, Pliny, and also by some 
mediaeval scientists, it was but natural that their demon¬ 
stration should follow, above all things, the construction 
and improvement of magnifying instruments. The honor 
of having been first to actually see and describe living 
organisms, too small to be visible to the naked eye, be¬ 
longs to a Jesuit priest, Kircher (1659), who was fol¬ 
lowed, a few years later, by the Dutch linen-draper, Leeu¬ 
wenhoek (1675). There is very little doubt, however, 
that by far the greater number of the “animalcules” seen 
by these observers were not bacteria but various protozoa 
(from Greek protos — first, and zoon = animal, the 
lowest form of animal life), as most of their studies were 
made on water. 
The period intervening between the end of the seven- 
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