Opinions are so firmly fixed at present that water is capable of carrying
the germs of disease that, in cases of epidemics, the recommendation is
made to drink natural mineral waters, or to boil ordinary water. This is a
wise measure, assuredly; but mineral waters are expensive, and, moreover,
many persons cannot get used to them. As for boiled water, that is a
beverage which has no longer a normal composition; a portion of its salts
has become precipitated, and its dissolved gases have been given off. In
spite of the aeration that it is afterward made to undergo, it preserves an
insipid taste, and I believe that it is not very digestible. I have
thought, then, that it would be important, from a hygienic standpoint, to
have a filter that should effectually rid water of all the microbes or
germs that it contains, while at the same time preserving the salts or
gases that it holds in solution. I have reached such a result, and,
although it is always delicate to speak of things that one has himself
done, I think the question is too important to allow me to hold back my
opinion in regard to the apparatus. It is a question of general hygiene
before which my own personality must disappear completely.

In Mr. Pasteur's laboratory, we filter the liquids in which microbes have
been cultivated, so as to separate them from the medium in which they
exist. For this purpose we employ a small unglazed porcelain tube that we
have had especially constructed therefor. The liquid traverses the porous
sides of this under the influence of atmospheric pressure, since we cause a
vacuum around the tube by means of an air-pump. We collect in this way,
after several hours, a few cubic inches of a liquid which is absolutely
pure, since animals may be inoculated with it without danger to them, while
the smallest quantity of the same liquid, when not filtered, infallibly
causes death.