vii INFECTION OF ANIMALS WITH PLAGUE 183 



of the B. pestis in soil, and had found this to be of very- 

 short duration. These experiments of Hankin have been 

 relied on by commentators as justifying the view that the 

 observed persistence of infectivity in rooms and streets 

 after removal therefrom of infected persons might have 

 been due to retention of the contagium by rats or by fleas ; 

 in a word, to the contagium having been kept alive as it 

 were by these means. 



Before, however, accepting such interpretation of the 

 facts it seemed to me necessary to ascertain whether the 

 vitality of B. pestis in soil is really of short duration ; 

 whether the result might not have been different if, 

 instead of distributing B. pestis in the soil, as practised 

 by Hankin, in the form of a broth culture, it had been 

 applied under conditions more nearly approaching those 

 in which it would naturally find access to soil, e.g. in 

 blood, in discharges of a suppurating bubo, in mucous 

 expectorations, in the mucoid discharges from the bowels 

 of a plague rat, in the juices of plague organs, or in 

 similar viscid or slimy materials. 



Accordingly I have made a series of experiments in 

 this direction, and have thereby ascertained that B. pestis 

 planted in or mixed with soil in the above manner has 

 considerable power of persistence. Also I have made 

 experiments with wheat and rice on the same lines ; 

 that is, I have sought to ascertain for what length of 

 time the bacilli in plague material (cultures or organs) 

 mixed with wheat or with rice can retain their vitality. 



Gelatine cultures, about six weeks old, were melted in 

 warm water and poured over fine earth (gravel) contained 

 in a glass dish (July 2), and over fine sand (July 1) con- 

 tained in a separate glass dish. The earth and the sand were 



