EXCEPTIONAL PHESOVESA. 339 



most familiar facts may teem with indications of forces, 

 now secrets hidden from us, because we have not mind- 

 directed eyes to discriminate them. The progress of 

 science will consist in the discovery from time to time 

 of new exceptional phenomena, and their assignment by 

 degrees to one or other of the heads already described. 

 When a new fact proves to be merely a false, apparent, 

 singular, divergent, or accidental exception, we may gain 

 a more minute and accurate acquaintance with the effects 

 of certain laws already known, to exist. We have indeed 

 no addition to what was implicitly in our possession, but, 

 as already explained, there is much difference between 

 knowing the laws of nature and perceiving all their com- 

 plicated effects. Should a new fact prove to be a limiting 

 or real exception, we have to alter, in part or in whole, 

 our views of nature and are saved from errors into which 

 we had fallen. Lastly, the new fact may come under the 

 sixth class, and may eventually prove to be a novel and 

 unexplained phenomenon, indicating the existence of 

 new laws and forces, complicating but not otherwise 

 interfering with the effects of laws and forces previously 

 known. 



The best instance which I can find of an unresolved ex- 

 ceptional phenomenon, consists in the anomalous vapour- 

 densities of phosphorus, arsenic, mercury, and cadmium. 

 It is one of the most important laws of chemistry, dis- 

 covered by Gay-Lussac, that equal volumes of gases exactly 

 correspond to equivalent weights of the substances, and 

 this holds generally true of any elements which we can 

 convert into gas or vapour. Unfortunately phosphorus 

 and arsenic give vapours exactly twice as dense as they 

 should do by analogy, and mercury and cadmium diverge 

 in the other direction, giving vapours half as dense as we 

 should expect. We cannot treat these anomalies as limit- 

 ing exceptions, and say that the law holds true of sub- 



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