1903-4. J ULTRAMICROSCOPIC ORGANISMS. 59 
It is probable that the virus of contagious pleuro-pneumonia does 
belong to the bacteria. It is, as I pointed out, just visible, and it is also 
cultivable outside the body. But this is the only one of this group, which 
I believe can be assigned to the bacteria. As we shall see, it is probably 
only slightly smaller than the width of half a wave length of the middle 
part of the spectrum, and consequently does not really differ much in size 
from the ordinary bacteria, and very slightly from such a minute form as 
the bacillus of influenza. bs 
But in regard to the truly ultramicroscopic forms, such as the virus 
of foot and mouth disease, the fowl plague of northern Italy, the African 
horse sickness and yellow fever, we are in the greatest uncertainty as 
to position. It is true that in regard to Beyerinck’s virus of the tobacco 
mosaic disease, he cuts the knot at once by calling it a contagium vivum 
fluidum, that is a living reproducing molecule or molecular complex so small 
and so simple that it is practically in solution in the fluids in which it is 
living and multiplying. There is nothing, a priori, to be urged against 
this view, but if such is the nature of these viruses, then we have to do 
with a new series of chemical compounds, with which we have as yet had 
no experience. 
The nearest approach to a condition of matter comparable to stich a 
living molecule is in the enzymes or unorganized ferments in the animal 
and plant body or the so-called catalytic agents, such as colloidal platinum. 
But these although active in most minute quantities, yet are definitely 
used up and cannot reproduce themselves. 
There is really no argument to offer against this view except that we 
naturally are loath to accept the existence of such a substance until we can 
more definitely prove it. If this should be the explanation, however, 
then we would be getting much closer to the hypothetical first form of 
life upon the earth in that the distance between a living reproducing mole- 
cule or molecular complex and a molecule of dead organic matter must 
be very short. 
The conditions which govern the destruction of these ultramicro- 
scopic parasites have been studied, viz., the thermal death point, and the 
effect of disinfectants, and it is found that they do not differ materially 
in susceptibility from the higher and larger bacteria. For instance, 
moist heat of 55° C. for ten minutes destroys the virus of yellow fever, 
but the spirillum of Asiatic cholera is destroyed by 52° C. for ten minutes. 
Solutions of fluid disinfectants such as carbolic acid and mercuric chloride 
act in the same way upon them as upon bacteria, but in this they resemble 
also the enzymes and even the inorganic catalysers, as Bredig has shown. 
There is only one fact which it seems to me rather militates against 
