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THE POPULAR SCIENCE MOXTIILY. 



This metliod is based essentially upon the following facts : 



Inoculation of a rabbit, by trepanning, under the dura mater, with 

 the poisonous marrow of a mad dog, always gives rabies to the animal 

 after a mean period of incubation of about fifteen days. If the virus 

 is passed from this first rabbit to a second, from this one to a third, 

 and so on, by the same method of inoculation, there is shortly mani- 

 fested a more and more marked tendency toward a shortening of the 

 period of incubation in the rabbits successively inoculated. After 

 from twenty to twenty-five passages from rabbit to rabbit, we arrive 

 at a period of incubation of eight days, which is maintained during a 

 new series of from twenty to twenty-five passages. Then we have a 

 period of incubation of seven days, which occurs with striking regu- 

 larity during a new series of passages rising to the ninetieth. At 

 least that is the number I have now reached without having hardly 

 yet observed a tendency to a slight further shortening of the period. 



The experiments of this character, which I began in November, 

 1882, have already been continued for three years without the series 

 having been interrupted, or without my having used any other virus 

 than that from rabbits Avhich successively died rabid. Nothing, there- 

 fore, is more easy than to have at one's disjiosition, during considerable 

 intervals of time, a virus of perfect purity, always identical, or nearly 

 so. This is the practical point of the method. 



The marrows of these rabbits are infected with rabies of constant 

 virulence in their whole extent. If we detach from them pieces a few 

 centimetres long, taking the greatest possible precautions to insure 

 their purity, and susjsend them in dry air, the virulence of the rabies 

 in them will slowly pass away, till it is quite extinguished. The dura- 

 tion of the process varies somewhat with the thickness of the marrow, 

 but depends chiefly on the exterior temperature : the lower the tem- 

 perature the longer the virulence lasts. These results constitute the 

 scientific point of the method.* 



These facts being substantiated, we have the following method of 

 making a dog, within a reasonably short time, proof against rabies. 



In a series of flasks, the air of which is kept dry by pieces of potash 

 in the bottom, we suspend each day a piece of freshly infected marrow 

 from a rabbit that has died of rabies, developed after seven days of 

 incubation. Every day, at the same time, we inoculate under the skin 

 of a dog a Pravaz syringeful of sterilized broth, in which has been 

 soaked a small piece of one of the marrows we arc keeping in desicca- 

 tion, beginning with one of those which we have prepared several days 

 before our operation is performed, so as to be sure that it is not of full 

 strength. On that subject we have informed ourselves by previous 

 experiments. We operate in the same manner on the following days 



* If the infected marrow is protected from the air, and is kept moist in carbonic acid, 

 the virulence will last, for several months at least, without change in intensity, provided 

 it be guarded ajrainst the attack of microbes from without. 



