J. Henderson Smith 



31 



These experiments have been frequently repeated and always with 

 the same general result. In experiments performed on different days 

 and with different spore cultures the minor details never completely 

 correspond, and it is almost impossible at a later date to reproduce 

 exactly the result obtained on a previous occasion. For this many 

 factors are responsible. The age of the culture and the number of spores 

 used are of great importance, as will appear later, but the chief source 

 of the discrepancies hes in the almost insuperable difficulty of obtaining 

 on different days suspensions of spores which are perfectly ahke. Small 

 differences in the degree of moisture in the culture tubes, the tightness 

 or looseness of the cotton wool plugs affecting the oxygen supply, and 



Survivors 



100 «^ 



Minutes 30 



180 



FiK. 1. 



the like, modify the rate and character of the development of the spores, 

 and this is reflected in the mortahty rate. But the general character 

 of the curves remains the same, a sigmoid curve with a stage of in- 

 creasing steepness, a maximum, and finally a stage of decreasing steep- 

 ness, flattening out more and more as time goes on. 



This is unhke the curve generally accepted as the typical mortahty 

 curve for bacteria exposed to disinfectant agents, such as phenol, cor- 

 rosive subhmate, heat, sunhght, etc. Since the papers of Chick in 

 England (3, 4) and Madsen and Nyman in Denmark (5), the typical 

 mortahty curve has been recognised as a logarithmic one, corresponding 

 in shape to the unimolecular curve, in which the number dying at any 



