n6 SCIENCE PROGRESS. 



hours at a temperature of io°-i2° C. with .75 per cent, potas- 

 sium hydrate. This treatment yields a viscid opalescent fluid 

 which is filtered through paper, and the precipitate obtained 

 from the filtrate by precipitation with weak acetic acid is 

 collected, washed, dried and dissolved in sodium carbonate. 

 Prepared in this way the solution probably contains among 

 other bodies a nucleo-proteid, and is found to be toxic for 

 rats and mice. The same substance if injected in non- 

 lethal doses, either subcutaneously or into the peritoneal 

 cavity, confers immunity to infection by virulent pest-bacilli, 

 and this condition lasts for about four weeks, and the serum 

 of the blood of these animals during this time possesses 

 both an immunising and protective power. In the method 

 devised by Roux the pest-bacillus is rendered exceed- 

 ingly virulent by introducing microbes enclosed in small 

 collodion bags into the peritoneal cavity of the rabbit. 

 Within the sacs a free development takes place, and the 

 bacilli acquire a high degree of virulence. If these are 

 now grown upon gelatine bouillon the toxic products of 

 the bacteria can be separated by filtration through a 

 Pasteur-Chamberland tube. A still more active toxin is 

 obtained if the bacilli are allowed to die in the culture fluid, 

 which then extracts the toxin. The clear fluid rich in 

 toxin is precipitated by ammonium sulphate, and the de- 

 posit collected and dried. The activity of this powder is 

 such that .25 milligrammes will kill mice in a few hours. 

 There is nothing of special interest connected with the toxin 

 of the plague-bacillus. It is obviously a mixture of several 

 substances, some of which are of a proteid character. The 

 chemistry of this, as of other toxins, is a matter of con- 

 jecture, and this is recognised only by its effects, for there 

 is no doubt that by small repeated injections a state of 

 immunity can be conferred on animals otherwise susceptible 

 to infection with living plague-bacilli. 



The prophylactic and curative power of the serum of 

 animals that have been experimentally brought to a high 

 grade of acquired immunity by the introduction of toxins, 

 dead bacilli or attenuated cultures into the system has 

 been extensively studied by Yersin (31), Haffkine (32), 



