through it. In laboratory uses, denser filters of smaller diameters
are used, and the filter is surrounded by the fluid to be tested. The
open end of the filter passes into a vessel from which the air is
exhausted and filtration takes place from without inward. The test of
the effectiveness of the filter is made by adding to the filtering
fluid some very minute and easily recognizable bacteria and testing
the filtrate for their presence. These filters have been studied
microscopically by grinding very thin sections and measuring the
diameter of the spaces in the material. These are very numerous, and
from 1/25000 to 1/1000 of an inch in diameter, spaces which would
allow bacteria to pass through, but they are held back by the very
fine openings between the spaces and by the tortuosity of the
intercommunications. When the coarser of such filters have been long
in domestic service in filtering drinking water, bacteria may grow in
and through them giving greater bacterial content to the supposed
bacteria-free filtrate than in the filtering water.

That an animal disease was due to such a minute and filterable
organism was first shown by Loeffler in 1898 for the foot and mouth
disease of cattle. This is one of the most infectious and easily
communicable diseases. The lesions of the disease take the form of
blisters which form on the lips and feet and in the mouths of cattle,
and inoculation with minute quantities of the fluid in the blisters
produces the disease. Loeffler filtered the fluid through porcelain
filters, hoping to obtain a material which inoculated into other
cattle would render them immune, and to his surprise found that the
typical disease was produced by inoculating with the filtrate.
Naturally the first idea was that the disease was caused by some
soluble poison and not by a living organism, but this was disproved in
a number of ways. The most powerful poison known is obtained from