470 INOCULATION FOR PLEURO-PNEUMONIA IN CATTLE. 
converted into the same description of virus as that which had 
been employed in the process, is not again susceptible, as a 
rule, to a second action of the morbific matter. In smallpox 
and other diseases of the class, we trace this immunity to the 
production of the malady by artificial means, and this pro- 
duction to an augmentation of its special virus within the 
organism. 
The protection afforded by vaccination to an individual against 
smallpox stands upon a foundation equally secure with that 
of inoculation. “ The vaccine disease 99 is in reality the small- 
pox of the cow. By the process which has been named 
vaccination the smallpox of the cow is transmitted to man, 
and fortunately with incalculable benefit, because while the 
disease possesses the same nature , it wants the malignancy of 
human smallpox. 
The experiments we have herein recorded prove that some 
of the animals were susceptible to the action of the serous 
exudations of the diseased lung, not only a second but a third 
time , or oftener . These cases are too numerous to be viewed 
simply as exceptions, for in our first experiments, out of seven 
selected animals, selected because of the success of the inoculation , 
no less than four were immediately acted on by a second 
inoculation. Nor do we stand alone in proving these facts; 
the Belgian Commissioners, as we have previously remarked, 
state i( that the phenomena succeeding inoculation may be 
produced several times in the same animal and they add, 
what is of equal importance, “ which may or may not have 
been attacked with exudative Pleuro-pneumonia.” 
We admit that the period of immunity afforded by inocula- 
tion as a general principle does vary in different diseases, and 
also in the same disease in different animals. What, w r e ask, 
is the proof of this immunity being lost? Why, the suscepti- 
bility of the animal to a re-inoculation. If this should fail, the 
animal was secure ; if it succeed, the animal w as unprotected. 
Among the cow 7 s w T e have alluded to w ere some which had 
been inoculated only thr 3e w T eeks before, and in others the 
effects of the first operation w T ere still manifest at the time of 
the successful re-inoculation . In our experiments also, when 
w r e succeeded in producing effects w 7 hich w r ould give perfect 
satisfaction to the advocates of the system, by inoculation 
with the serous fluid , we have taken the product of this 
inoculation and used it forthwith on the self-same animal as 
well as on others , and obtained results equally as great as from 
the original inoculation. How could these things be explained 
were we dealing with a specific virus ? 
We have called this product a sero purulent fluid, and so in 
