92 SUMMARY OF CUKEENT RESEARCHES RELATING TO 



death occurs, and in tlie diminished acuteness of the preliminary 

 symptoms. 



On the other hand, with longer intervals than two months, these 

 indications become decided, so much so that the percentage of deaths 

 falls from that of 10 in 10 cases, which occurred in the former experi- 

 ments, to as low as 1 in 10 ; and sometimes no deaths take place. 

 Thus a completely vaccinal virus may be obtained. 



The rules to which the diminution of the virulence of the poison 

 is subject are not mathematically exact ; that is, the diminution of 

 intensity does not vary directly as the length of the interval allowed 

 to the growth of the parasite ; but it is certain that death of the 

 parasite ensues sooner or later if sufficient time is allowed, and fur- 

 ther, that the standard of virulence attained by any solution is main- 

 tained by it so long as the periods of growth are kept constant. 



No changes in the appearance of the parasite itself have hitherto 

 been found to accompany the changes in its activity. 



With regard to the cause of the diminution of virulence, the fact 

 that the parasite requires contact with the air, and fails to develoi? in 

 a solution if this is denied it, suggests the possibility that this contact 

 with the air or with some chemical principle contained in it is at the 

 bottom of this remarkable change. By removing the oxygen it may 

 be shown that it is the agent which effects this change. This is 

 done by sealing up tubes containing infected solutions, with various 

 proportions of air. At first the solutions become cloudy, marking the 

 growth of the organism. In two or three days they become clear, and 

 the growth is deposited on the sides of the glass. The cause of the 

 clearing is the cessation of growth consequent on the exhaustion of 

 the small amount of oxygen belonging to the enclosed air. On open- 

 ing these tubes at periods of from two to ten months after sealing, the 

 poison is found to retain its power, which is exactly of the degree 

 at which it stood when first sealed up, whereas growths exposed to the 

 air at the same time are now either dead or weakened in force. 



It remains to be seen whether this action of the oxygen similarly 

 affects other organized poisons, in order to deduce a principle from the 

 facts. If it is so, the limitation which great epidemics undergo may 

 here find an explanation. It is noteworthy that the above explanation 

 was arrived at in spite of a former impression formed by M. Pasteur 

 that the facts relating to the attenuation of the poison were due to the 

 existence of two different kinds of the poison, one very virulent, the 

 other very feeble. 



Influence exerted on the Bacterium of Splenic Fever by Sub- 

 jects refractory to it.* — M. A. Chauveau has tested this question by 

 taking sheep which were known to be naturally refractory to the 

 poison, and reinforcing this natural inaptitude by inoculating, so as to 

 produce almost complete protection, and then injecting infected blood 

 into their circulation in large quantities. Thus eight sheei) were sub- 

 jected to several preliminary inoculations by hypodermic injections, 

 three of them having, besides, 1 c.c. of infected blood injected into the 



* Comptes Rendus, xci. (1880) pp. 680-4. 



