324 THE PRINCIPLES OF SCIENCE. 



Generalization, (vol. ii. p. 259). Quicksilver is divergent 

 among metals as regards its melting point, and potassium 

 and sodium as regards their specific gravity. Monstrous 

 productions and variations, whether in the animal or 

 vegetable kingdoms, should probably be assigned to this 

 class of exceptions. 



Accidental Exceptions. 



The third and largest class of exceptions contains those 

 which arise from the casual interference of extraneous causes. 

 A law may be in operation, and, if so, must be perfectly 

 fulfilled, but, while we conceive that we are examining 

 its results, we may have before us the effects of a totally 

 different cause, possessing no connexion with the subject 

 of our inquiry. The law is not really broken, but at the 

 same time the supposed exception is not illusory. It may 

 be a phenomenon which cannot occur but under the con 

 dition of the law in question, yet there has been such 

 subsequent interference and modification of the result, 

 that there is an apparent failure of science. There is, 

 for instance, no subject in which more rigorous and in 

 variable laws have been established than in crystallo 

 graphy. As a general rule, each chemical substance pos 

 sesses its own definite form, by which it can be infallibly 

 recognised ; but the mineralogist has to be on his guard 

 against what are called pseudomorphic crystals. In some 

 circumstances a substance, having perfectly assumed its 

 proper crystalline form, may afterwards undergo chemi 

 cal change ; a new ingredient may be added, a former 

 one removed, or one element may be substituted for 

 another. In carbonate of lime the carbonic acid is some 

 times replaced by sulphuric acid, so that we find gypsum 

 in the form of calcite ; other cases are known where the 

 change is inverted and calcite is found in the form of 

 gypsum. Mica, talc, steatite, hematite, are other minerals 



