THE SCIENTIFIC MOOD. 23 



more data, but for putting our hypothesis to the 

 proof. We allude to what are called test or crucial 

 experiments and control experiments. Much of the 

 success of a scientific worker may depend on his 

 ingenuity in thinking out crucial experiments and 

 on his rigorous use of control experiments. 



When bacteriology was in its infancy, Pasteur put 

 his theory that putrefaction was the result of the life 

 of micro-organisms to a crucial test when he steri- 

 lised readily putrescible substances, and, having her- 

 metically sealed the vessel, kept them for years with- 

 out the occurrence of any putrefaction. 



When Von Siebold and his fellow-workers had 

 gradually convinced themselves that certain bladder- 

 worms in various animals used as food were the 

 young stages of certain tapeworms occurring in man, 

 they made the crucial experiment of swallowing the 

 bladderworms and proved the accuracy of their con- 

 clusion by becoming shortly afterwards infected with 

 tapeworm. 



The control experiment is closely akin. A cray- 

 fish is known to have a sense of smell. Various rea- 

 Bons lead the enquirer to conclude that this sense has 

 its seat in the antennules. He may verify this by 

 observing that a crayfish without these appendages 

 will not respond to a strong odour, but he would not 

 be satisfied unless he had shown that in exactly the 

 same conditions and to exactly the same stimulus an- 

 other crayfish with its antennules intact did actively 

 respond. Having gone so far, he would proceed to 

 localise the sense more precisely; microscopic re- 

 search would direct his attention to peculiarly shaped 

 bristles on the antennules. By shaving these off, and 

 observing that response to strong odours ceased, he 

 would prove his point, but again, in view of possible 



