280 ORIENTAL PLAGUE chap. 



watery emulsions of cultures, or prepared direct from broth 

 cultures, are exposed to drying such as can be effected at 

 45° to 46° C. in less thau twenty-four hours, at 37° C. 

 in twenty - four hours, over sulphuric acid at ordinary 

 temperature in forty-eight hours, the microbes of such 

 films become thereby quite devitalised ; for if the cover 

 films are, after drying, placed in nutrient broth and in- 

 cubated, no growth of B. pestis takes place. It stands 

 to reason, and direct experiment proves it, that the 

 period of drying of B. pestis in a viscid material — e.g. 

 blood, spleen tissue, gelatine culture (melted in warm 

 water) — requires to be considerably extended. Take, for 

 instance, the B. pestis in blood or spleen tissue exposed 

 to 46° C. even over sulphuric acid. Under these con- 

 ditions the B. pestis cannot with certainty be considered 

 dead after twenty-four hours. I have had cases when 

 even after thirty-six hours not all B. pestis were dead, 

 though three days of such drying can be fully relied on. 



2. Spontaneous. — It has been pointed out in a former 

 chapter that transferring B. pestis from one culture to 

 another its virulence gradually diminishes ; this is a fact 

 also observed with other microbes. As regards B. pestis 

 we have pointed out an important difference, viz. this, 

 that while some strains lose in artificial cultures their 

 virulence slowly, others do so very rapidly, even with 

 complete extinction of virulence. We have shown that 

 in this respect the rat type plague bacillus (type 2), at 

 starting already endowed with lesser virulence than the 

 human type (type 1), is apt to lose its virulence, in a 

 relatively very short time, and not only to become less 

 and less virulent, but to become ultimately a quite harmless 

 saprophyte. Might not this find its counterpart under 



