differ greatly in appearance from the
elongated dot of the bacterium proper, to the elongated rod or
cylinder of the bacillus, and the long spirals of spiro-bacteria. It
is unfortunate that they are not sufficiently constant in habit to
always attach themselves to one or the other of these genera. The
micrococcus has a habit of elongating at times until it is impossible
to recognize him except as a bacterium; while bacilli, again, break up
until their particles exactly resemble micrococci.

Bacteria cannot exist without water; certain forms require oxygen,
while others thrive equally well without it; some thrive in solution
of simple salts, while others require albuminoid material.

Bacteriology, with its relation to the science of medicine, is of
importance to every investigating physician; it covers our knowledge
of the relation of these minute organisms to the aetiology of disease.
What has been gained as to practical application in the treatment of
disease? This question is not infrequently asked in a sneering manner.
We can, in reply, say that the results are not all in the future. It
is encouraging that results have been attained which have had a very
important practical bearing, and that these complaints come generally
from individuals least acquainted with scientific investigations in
bacteriology.

In the study of the relation of a given bacterium to a certain
disease, it becomes necessary to attend carefully to three different
operations: First, the organism supposed to cause the disease must be
found and isolated. Second, it must be cultivated through several
generations in order that absolute purity may be secured. Lastly, the
germ must be again introduced into a healthy living being. If the
preceding steps be carried out, and the original disease be
communicated by inoculation, and the germs be again found in the
diseased body, we have no alternative; we must conclude that we have
ascertained the cause of the disease. The importance of being familiar
with the aetiology of the disease before we can expect to combat it
with any well-grounded hope of success is evident.

If the sputum of a phthisical patient be submitted to the skilled
microscopist, he is nearly always able to demonstrate bacilli, but
this goes for very little. Because bacilli are found in phthisis, it
is no more certain that they are the cause of phthisis than it is
certain that cheese mites are the cause of cheese. Well, suppose we
wer