54 TRANSACTIONS OF THE CANADIAN INSTITUTE. VoL. VIII. 
filter out the vast majority of known bacteria and which have been used 
regularly for this purpose for many years. ‘These filters are all of the same 
type, in that they have pores so small that bacteria cannot be washed 
through them. The first to be manufactured consisted of an unglazed porce- 
lain and was called the Pasteur-Chamberland filter, but there are now a 
number of different forms such as the Berkefeld which consists of compres- 
sed infusorial earth, and which are equally successfully. The impermeabil- 
ity of a Pasteur-Chamberland or a Berkefeld filter to bacteria is due to 
the fact that the minute passages or pores are not only small but tortuous 
and consequently the first organisms which pass on to the surface or into 
the mouths of the pores are caught and form a film which assists in render- 
ing the filters even more effective. On the other hand it has been shown 
that many bacteria which cannot be driven through these filters by pressure 
can grow through their walls if given time and especially if they have the 
power of independent motion. ‘The effectiveness of the filter is therefore 
due to the thickness of the walls and the tortuosity of the passages as well 
as to their minuteness. 
An organism which could readily pass through such a filter would 
probably be of ultramicroscopic size and it has consequently been by a 
combination of the method of animal inoculation and filtration through 
such a filter that these organisms have been discovered. 
Practically the first knowledge which we have of such minute living 
creatures resulted from the work of Loeffler and Frosch upon foot and 
mouth disease. These investigators undertook for the German Govern- 
ment a study of this disease which is a serious menace to the stock raising 
industry of various parts of the world and has for the human ‘race the 
additional interest in that it is communicable to man. 
In this disease, the characteristic feature is the presence of small 
vesicles or blebs upon the mucous membrane of the mouth and lips, and 
also about the hoofs of the forefeet. The eruption of vesicles is accom- 
panied by more or less constitutional evidence of disease. The disease 
is exceedingly contagious, sweeping through a herd or from herd to herd 
with great rapidity. The vesicles on puncture yield a small amount of 
clear serous fluid and Loeffler and his colleague found that the contagion, 
whatever it was, was present in this serum. Microscopic examination 
of this fluid did not show anything which could be interpreted as a living 
organism although a minute quantity of it was sufficient to infect another 
animal. They therefore assumed that it was probably ultramicroscopic 
and endeavoured to see if it would pass through the pores of a Pasteur 
filter. Their results showed that the fluid is equally contagious after passing 
through the filter and they therefore concluded that it was smaller than 
any known organism. 
