the Etiology of rabies. 
223 
ords by the process of desiccation is explained on the supposition 
tiat the continuous contact of the dry air produces a gradual 
iminution in the intensity of the virulence of the cords until it 
nally becomes extinct; that the prophylactic method in its ap- 
lication depends for its efficacy upon the employment at first of 
virus without appreciable activity, followed by a weak virus, 
ad then by a more and more virulent form; that the diminution 
f the virulence of the cords is due to an impoverishment in 
uantity of the virus contained in them, and not to an impover- 
hment in virulence; consequently in the inoculations the virus 
sed is always identical as regards its virulence, and is variable 
! aly in respect to the quantity employed, so that the refractory 
mdition to rabies follows from the employment of very small 
at constantly increasing quantities of a virus possessing always 
le same degree of virulence. This interpretation of the method 
F action of the virus is the more interesting as a new and quite 
ifferent principle is involved from that obtaining in the vaccine 
>r small-pox or the vaccines devised by Pasteur for the preven- 
on of anthrax, chicken-cholera, and typhus in pigs. In small- 
dx we have a virus modified in character and virulence by its 
issage through another species of animal; in anthrax, chicken- 
lolera, and typhus in pigs, we have a virus modified in respect 
* its virulence by the conditions of temperature to which it has 
3en subjected during its growth; and, finally, in rabies we have 
prophylactic method dependent upon the employment of a virus 
ways constant as regards its virulence, but used in very small 
id constantly increasing quantities. Apparently, then, already 
I iere have been three methods discovered for the preparation of 
•eventive vaccines for different forms of the contagious diseases, 
lie theoretical principle has been also established by the 
searches of Pasteur, and more recently by Burdon Sanderson, 
e report of whose investigations has not yet been published, of 
e possibility of depriving cultivations of pathogenic miero- 
ganisms of their virulence in a constant and persistent degree 
ithout in any other respect affecting their morphological or bio- 
gical characteristics. These methods for preventive vaccination 
rtaken in connection with this principle, which is involved in 
