xxix.] EXCEPTIONAL PHENOMENA. 667 



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 abhorrence decrease and 

 finally disappear altogether. Even Aristotelian doctrines 

 could not stand such direct contradiction. 



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 generalisation 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 exception, but 

 when so powerful an acid as hydrogen chloride (muriatic 

 acid) was found to contain no oxygen the theory had to be 

 relinquished. Berzelius' theory of the dual formation of 

 chemical compounds met a similar fate. 



It is obvious that all conclusive experimenta crucis con- 

 stitute real exceptions to the supposed laws of the theory 

 which is overthrown. Newton's corpuscular theory of light 

 was not rejected on account of its absurdity or incon- 

 ceivability, for in these respects it is, as we have seen, far 

 superior to the undulatory theory. It was rejected because 

 certain small fringes of colour did not appear in the exact 

 place and of the exact size in which calculation showed 

 that they ought to appear according to the theory (pp. 5 16- 

 521). One single fact clearly irreconcilable with a theory 

 involves its rejection. In the greater number of cases, 

 what appears to be a fatal exception may be afterwards 

 explained away as a singular or disguised result of the 

 laws with which it seems to conflict, or as due to the inter- 

 ference of extraneous causes ; but if we fail thus to reduce 

 the fact to congruity, it remains more powerful than any 

 theories or any dogmas. 



Of late years not a few of the favourite doctrines of 

 geologists have been rudely destroyed. It was the general 

 belief that human remains were to be found only in those 

 deposits which are actually in progress at the present day, 

 so that the creation of man appeared to have taken place 

 in this geological age. The discovery of a single worked 

 flint in older strata and in connexion with the remains of 

 extinct mammals was sufficient to explode such a doctrine. 



