336 THE PRINCIPLES OF SCIENCE. 



Real Exceptions to Supposed Laws. 



The exceptions which we have lastly to consider, are 

 perhaps the most important of all, since they lead to 

 the entire rejection of a law or theory before accepted. 

 No law of nature can fail ; there are no such things as 

 real exceptions. Where contradiction exists it must be in 

 the mind of the experimentalist. Either the law is 

 imaginary or the phenomena which conflict with it ; if, 

 then, by our senses we can satisfy ourselves of the actual 

 occurrence of the phenomena, the law must be rejected 

 as illusory. The followers of Aristotle held that nature 

 abhorred a vacuum, and thus accounted for the rise of 

 water in a pump. When Torricelli pointed out the visible 

 fact that water would not rise more than 33 feet in a 

 pump, nor mercury more than about 30 inches in a glass 

 tube, they attempted to represent these facts as limiting 

 exceptions, saying that nature abhorred a vacuum to a 

 certain extent and no further. But the Academicians 

 del Cimento completed their discomfiture by showing that 

 if we remove the pressure of the surrounding air, and 

 in proportion as we remove it, nature's feelings of abhor- 

 rence decrease and finally disappear altogether. Even 

 Aristotelian doctrines could not stand such direct contra- 

 diction. 



Lavoisier's ideas concerning the constitution of acids 

 received complete refutation. He named oxygen the acid 

 generator, because he believed that all acids were com- 

 pounds of oxygen, a generalization based on insufficient 

 data. Berthollet, as early as 1789, proved by analysis that 

 hydrogen sulphide and prussic acid, both clearly acting 

 the part of acids, were devoid of oxygen ; the former 

 might perhaps have been interpreted as a limiting excep- 

 tion, but when so powerful an acid as hydrogen chloride 



