[22 POPULAR SCIENCE MONTHLY. 



pregnanl female were found dead, while twenty-four out of the twenty- 

 five that had been vaccinated wore perfectly well, and exhibited during 

 the whole lime iliev were kept under observation the same degree of 

 health as the ten sheep that had. been put aside for comparison. 



An impetus was given by these discoveries to researches having for 

 their object the protection of men against infectious diseases. The 

 most important of these researches was Pasteur's own into the nature 

 of hydrophobia and rabies, and the way of inoculating against that 

 disease. This was followed a few years later by the preparation of a 

 prophylactic against cholera. 



Inoculation against hydrophobia was rendered possible by the dis- 

 covery of the fact that the rabies or hydrophobia virus is found in 

 a pure condition, free from other microbes, in the nervous centers of 

 animals. The material for inoculation is prepared from such nervous 

 matter, the virulence of which is rendered fixe, as will be mentioned 

 below. 



The cholera microbe, which was subsequently named comma bacillus, 

 was discovered by Koch in- 1883, in the intestinal contents of cholera 

 patients. Two years later cholera broke out in Spain, and Dr. James 

 Ferran, a Spanish physician, began inoculating men with living cultures 

 of comma bacillus taken from patients attacked with the disease. The 

 procedure in its essential features corresponded to the pre-Jenner 

 method of inoculation. The failure to fix the strength of the virus 

 used for treatment rendered the method subject to the same un- 

 certainty as thai which was connected with inoculation with small- 

 pox virus taken direct from patients. It was impossible to predict the 

 effect of the injections. Comma bacilli taken from cholera patients 

 may, under cultivation, show themselves extremely virulent, or, on the 

 contrary, extremely mild. There are specimens which, when injected 

 into a Guinea pig, even in an insignificant dose, will prove fatal to it, 

 and there are others which will appear harmless when given in a dose 

 seventy times greater. The immediate effect, and the protection caused 

 by the inoculation, must, of course, vary accordingly. The attempt 

 made by Ferran caused great interest, and a number of scientific com- 

 missions were sent to Spain from different countries of Europe to 

 study the results of his work. They could, however, come to no con- 

 clusion, and the treatment speedily lost its position. Only some seven 

 years later a method was found of fixing the strength of the cholera 

 virus. I was connected with this stage of the work, and it may per- 

 haps present some interesl to the reader to relate the way in which 

 the problem was solved, and to show how gradual is the development 

 of ideas by which results in laboratory investigation are arrived at. 



It has been mentioned already that the virulence of microbes changes 



