PART FIRST. 
MORPHOLOGY OF THE BACTERIA. 
CHAPTER I. 
ORGANIZATION OF THE BACTERIA. 
WueEN bacteria develop in-a liquid in a suffi- 
cient quantity, they become visible to the naked 
eye. They appear either as a slight cloud, or 
gathered in little masses in the liquid, or forming 
a pellicle upon its surface, or as a deposit upon the 
walls of the vessel and upon the objects contained 
in the liquid. However, we must hasten to say 
with M. Cohn, that the fact of the absence of all 
turbidity im a liquid does not exclude the possi- 
bility of the presence of bacteria. In liquids more 
dense than water (serum, lymph, etc.), when the 
refractive power of these corpuscles is the same as 
that of the liquid, their presence may not be 
revealed by the naked eye. We will add that 
sometimes their color serves to indicate their 
presence in a liquid, although this color is often 
very feeble, and can only be perceived when a 
considerable thickness of the liquid is examined. 
If we examine these clouds, these accumulations, 
these deposits, with the microscope, we see that 
