Ill PASTEUR AND HYDROPHOBIA 159 



ascertain, if such a conception is to be applied to 

 Pasteur's treatment of hydrophobia, is whether the 

 dog's and wolf's virus is longer in incubation and 

 stronger in poisonous quality than that of the rabbit's 

 cords as modified by hanging up in dry air. A general 

 principle appears to be according to M. Pasteur 

 that, in regard to rabies, the longer the incubation 

 period the less the virulence of the virus, and the 

 shorter the incubation period the greater the virulence. 

 The virus in the cord of the rabbits used by M. Pasteur 

 for preventive inoculation is stated by him to be, 

 when fresh, much more intense than that taken from 

 a mad dog ; it produces rabies in a dog, when injected 

 into its veins, in eight or ten days. By hanging in 

 dry air for a fortnight this cord loses its virulence. 

 But it has not yet been stated by Pasteur what are 

 the indications that this virulence is lost, and whether 

 the loss of " virulence " is in this case measured by 

 an increase of incubation period. We have no infor- 

 mation from Pasteur on this point. It would certainly 

 seem that the virus of the dried rabbits' cords ought 

 not to lose its short incubation period if it is to get 

 beforehand with the dog-bite virus, which has a period 

 of five or six weeks. 1 And presumably, therefore, 

 there must be two distinct qualities in which the 



1 The incubation period of five weeks ordinarily observed in the 

 case of men bitten by rabid dogs may be due to the smallness of the 

 dose, since Pasteur has shown that small doses of rabid virus give 

 longer incubation periods than large doses. How far a dose of 

 weakened virus can be made to attain the rapid action of strong virus, 

 by increasing the quantity of the weaker virus injected, has not been 

 stated by Pasteur. 



