664 APPENDIX. 
reached. If successive inoculations be made into rab- 
bits with fluid, either from the dog or the monkey, the 
virulence may be so exalted beyond that of the virus 
taken from a street dog, in which the incubation period 
is from twelve to fourteen days, that at the end of the 
one hundredth passage the incubation period may be 
reduced to about six or seven days. This, the strong- 
est virus obtainable, was called by Pasteur the fixed 
virus. Rabic virus appears also to become attenuated 
under certain conditions of temperature, and if it be 
subjected for about an hour to a temperature of 50° C. 
its activity is completely destroyed, or in half an hour 
if to a temperature of 60° C. A 5 per cent. solution 
of carbolic acid, acting for the same period, exerts a 
similar effect, as do likewise 1:1000 solutions of 
bichloride of mercury, acetic acid, or potassium per- 
manganate. The virus also rapidly loses its strength 
by exposure to air, especially in sunlight; when pro- 
tected from heat, light, and air it retains its virulence 
for a long period. In his earlier experiments Pasteur 
selected a series of rabic poisons of different strengths, 
beginning with that obtained from the spinal cord of 
the monkey—from the very weak to the strongest that 
he could obtain in this animal—then passing through a 
similar series obtained during the process of exaltation 
of the virus by passage through the rabbit. By inoc- 
ulating dogs subcutaneously with virus taken from a 
series commencing with the weakest taken from a mon- 
key, and gradually working up to that obtained from 
the rabbit—from the earliest to the latest in the series 
—the animals become immune not only against subcu- 
taneous injection but against subdural injection with 
fixed virus, and also against the bite of rabid dogs. 
