56 TRANSACTIONS OF THE CANADIAN INSTITUTE. VoL. VIII. 
juice was equally virulent, and therefore concluded that here also we were 
dealing with an ultramicroscopic virus. He went, however, a step further, 
and showed what was still more wonderful—that the contagion was diffusible 
—that it would pass by diffusion through a layer of agar jelly, just as a 
salt in solution might pass, and he therefore announced that he had discoy- 
ered a contagium vivum fluidum or, really, a soluble toxine, capable of 
reproduction. 
Another animal disease, which for some years baffled investigators, 
was a type of chicken disease which prevails in northern Italy. It was 
for a long time confused with the co-called chicken cholera, which is due 
to a well known bacillus isolated by Pasteur. But further study sufficed 
to separate it from this disease, and numerous attempts were made, without 
success, to cultivate from injected birds a specific micro-organism. 
Within the past year, the discovery has been made by Centanni and 
by two Austrian observers, independently, that in this particular disease, 
we have to do with a filterable virus. The blood of the animals contains 
the virus, and it is intensely virulent. A needle dipped in the infected 
blood, wiped off, and inserted beneath the skin of a healthy fowl, leads to 
its death in about thirty hours. This infected blood, when filtered through 
the densest of the Pasteur filters, does not show the slightest diminution 
in its virulence, and yet microscopic observation fails to reveal anything. 
Here again we have an example of a parasite of ultramicroscopic size. 
There are several other diseases of domestic animals which, as a result 
of the filtration test, combined with the failure to demonstrate organisms 
by the high magnifications, are to be classed as due to ultramicroscopic 
organisms. One of the most important, recently discovered, is the African 
Horse Sickness, which McFadyen, in 1900, showed was due to a filterable 
virus. ‘This virus, not only passes freely through the Pasteur filter, mark F, 
but even through the most compact form, mark B, which will hold back 
the virus of foot and mouth disease. 
But undoubtedly the most important of all of these ultramicroscopic 
viruses, as far as man is concerned, is that of yellow fever. 
It has this additional scientific interest that its demonstration has 
been of the most complete character, and wonderful practical results have 
flowed from the careful study of the conditions of transmission. 
Our present complete knowledge of yellow fever we owe to the late 
Major Reed, of the U.S. Army Medical Service, one of the most eminent 
and reliable of American bacteriologists, and his associates—Carroll and 
Agramonte. c 
Yellow fever is a disease which has been studied with the greatest 
assiduity ever since the development of bacteriological methods, and 
