ee ee ee ee es | he a le ee, ee, ee ee - eo = 
bi " t det Lah 
344 
comparatively long time in the dry state. Our experiments with variola 
virus are not very extensive and we hesitate to draw broad conclusions 
from them, but so far as they go they show that variola virus is less 
resistant to drying than vaccine. 'This whole question of the comparative 
resistance of the two sorts of virus to physical conditions is unsettled 
and presents an attractive field for study. 
In our earlier experiments we found that variola virus did not retain 
its potency in the presence of 60 per cent glycerin. As the results of 
our later experiments upon this point show variola virus will resist the 
action of glycerin for a considerable period, we are inclined to believe 
that our first, experiments were done with a glycerin which was not 
absolutely neutral, but that the inactivation of the virus was due to 
an acid in the glycerin and not to the action of the pure reagent. The 
loss of the power to develop an exanthem after prolonged contact with 
glycerin is of interest and should be made the basis of further work. 
It is important to determine whether this loss is associated with any 
change in the development of the two cycles of the parasite. 
The question of the passage of the variola and vaccine contagium 
through the filter has been the subject of a certain number of experiments. 
We have only one observation to record upon this, which shows that the 
organism is held back by the filter. It is to be noted that this inactiva- 
tion of the virus by filtration was tested by skin inoculations on the 
monkey and did not exclude the presence of diffusible contagium in the 
filtrate. 
The influence of serial transfers upon the variola virus is shown in 
our series. This demonstrates that the virus tends to die out when 
transferred from monkey to monkey. This is in contrast with the 
stability of a strain of vaccine on an animal. 
The question of the relative virulence of different strains of virus 
requires for its intelligent discussion the criteria for judging of the 
virulence of a given virus. In the case of variola virus the reactions of 
the animal inoculated consist in the development of a local lesion, the 
production of an exanthem, the tumefaction of the lymph nodes, and 
in some constitutional disturbance. The two former present many 
degrees of variation and seem most suitable as a basis for judgment as 
to the virulence of the virus inoculated. We can distinguish between 
various reactions at the site of inoculation and classify them as typical, 
atypical, abortive, ete. We can observe the occurrence, the extent, and 
the evolution of the exanthem. If we had only the virus to consider, 
we should classify our different strains or samples of virus according to 
the degree of local reaction at the site of inoculation and the nature of 
the exanthem. However, such a direct interpretation is impossible, as 
we must take into consideration the degree of natural immunity of 
the animal chosen for inoculation to a virus. In order to estimate this 
element it is necessary to compare the results of the inoculation of 
