EXCEPTIONAL PllKSnMEXA. 331 



degree by rays of light falling upon the selenium, points 

 to a wholly new relation between light and electricity. 

 The peculiar so-called allotropic changes which sulphur, 

 selenium, and phosphorus undergo by an alteration in 

 the amount of latent heat which they contain, will pro- 

 bably 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 sub- 

 stance m , which still, however, remains exceptional in its 

 properties and relations, and affords a hope of important 

 discoveries in chemical theory. 



Limiting Exceptions. 



We may pass to cases where exceptional phenomena 

 are actually irreconcilable with a law of nature previously 

 regarded as true by philosophers. Error must now be 

 allowed to have been committed, but it is obvious that 

 the error may be more or less extensive. It may be that 

 a law holding rigorously true of the facts actually under 

 notice had been extended by generalization to other 

 series of facts then unexamined. Subsequent investiga- 

 tion may show the falsity of this generalization, and 

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

 those objects of which it is really true, while we bring 

 the other classes of objects under distinct generalizations. 

 The contradiction to our previous opinions is partial and 

 not total. 



Xewton laid down as a result of experiment that eveiy 

 ray of homogeneous light has a definite refrangibility, which 



m ' Philosophical Transactions ' (1872), vol. clxii. 110. 23. 



