SCIENTIFIC METHOD 71 



ing out these crucial or test experiments. Let us 

 notice two or three examples. 



When bacteriology was still in its infancy, and 

 Pasteur was still fighting for his discovery that 

 putrefaction was due to the life of micro-organ- 

 isms in the rotting substance, he put his theory 

 to a crucial test which is continually repeated 

 now-a-days as a class experiment or for practical 

 purposes in the preservation of various foods. He 

 took some readily putrescible substances, steri- 

 lized them by boiling, and hermetically sealed 

 the vessel. No putrefaction occurred. 



When Von Siebold and his fellow-workers 

 had convinced themselves indirectly that certain 

 bladderworms, e. g. those which occur in the pig 

 and the ox, were the young stages of certain 

 tapeworms which occur in man, they made the 

 crucial and almost heroic experiment of swallow- 

 ing the bladderworms. By becoming soon after- 

 wards infected with the tapeworms they proved 

 the truth of their theory. 



Or let us take a simple case where the method 

 of exclusion is combined with a control experi- 

 ment. The freshwater crayfish has a sense of 

 smell, as is proved by the rapid way in which it 

 retreats from strong odours. Investigation led 

 to the hypothesis that this sense was located in 

 the antennules or smaller feelers. This was 

 verified by observing that a crayfish bereft of 



