658 THE PRINCIPLES OF SCIENCE. [CHAP. 



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 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 condition of the law in question, yet there has been 

 such interference that there is an apparent failure of 

 science. There is, for instance, no subject in which more 

 rigorous and invariable laws have been established than in 

 crystallography. As a general rule, each chemical sub- 

 stance possesses 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 assumed its proper 

 crystalline form, may afterwards undergo chemical change ; 

 a new ingredient may be added, a former one removed, or 

 one element may be substituted for another. In calcium 

 carbonate the carbonic acid is sometimes 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 subject to these curious 

 transmutations. Sometimes a crystal embedded in a matrix 

 is entirely dissolved away, and a new mineral is subse- 

 quently deposited in the cavity as in a mould. Quartz is 

 thus found cast in many forms wholly unnatural to it. A 

 still more perplexing case sometimes, occurs. Calcium 

 carbonate is capable of assuming two distinct forms of 

 crystallisation, in which it bears respectively the names of 

 calcite and arragonite. Now arragonite, while retaining its 

 outward form unchanged, may undergo an internal mole- 

 cular change into calcite, as indicated by the altered 

 cleavage. Thus we may come across crystals apparently 

 of arragonite, which seem to break all the laws of crystallo- 

 graphy, by possessing the cleavage of a different system of 

 crystallisation. 



