xxix.] EXCEPTIONAL PHENOMENA. 663 



phenomena will be discovered, and will lead to new views 

 of nature. The recent observation, for instance, that the 

 resistance of a bar of selenium to a current of electricity is 

 affected in an extraordinary degree by rays of light falling 

 upon the selenium, points to a new relation between light 

 and electricity. The allotropic changes which sulphur, 

 selenium, and phosphorus undergo by an alteration in the 

 amount of latent heat which they contain, will probably 

 lead at some future time to important inferences concerning 

 the molecular constitution of solids and liquids. The 

 curious substance ozone has perplexed many chemists, and 

 Andrews and Tait thought that it afforded evidence of the 

 decomposition of oxygen by the electric discharge. The 

 researches of Sir B. C. Brodie negative this notion, and afford 

 evidence of the real constitution of the substance, 1 which 

 still, however, remains exceptional in its properties and 

 relations, and affords a hope of important discoveries in 

 chemical theory. 



Limiting Exceptions. 



"We pass to cases where exceptional phenomena are 

 actually irreconcilable with a law of nature previously 

 regarded as true. Error must now be allowed to have been 

 committed, but the error may be more or less extensive. 

 It may happen that a law holding rigorously true of the 

 facts actually under notice had been extended by generalisa- 

 tion to other series of facts then unexamined. Subsequent 

 investigation may show the falsity of this generalisation, 

 and the result must be to limit the law for the future to 

 those objects of which it is really true. The contradiction 

 to our previous opinions is partial and not total. 



Newton laid down as a result of experiment that every 

 ray of homogeneous light has a definite refrangibility, which 

 it preserves throughout its course until extinguished. This 

 is one case of the general principle of undulatory movement, 

 which Herschel stated under the title " Principle of Forced 

 Vibrations" (p. 451), and asserted to be absolutely without 

 exception. But Herschel himself described in the Philo- 

 sophical Transactions for 1845 a curious appearance in a 



* Philosophical Transactions (1872), vol. clxii. No. 23. 



