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. 



