330 THE PRINCIPLES OF SCIENCE. 



and 12 of carbon. But when careful analyses were made 

 of a great many minerals, this law often appeared to fail. 

 What was unquestionably the same mineral, judging by 

 its crystalline form and physical properties, would often 

 give varying proportions of its components, and would 

 sometimes contain unusual elements which yet could not 

 be set down as mere impurities. Dolomite, for instance, is a 

 compound of the carbonates of magnesia and lime, but speci- 

 mens from different places do not exhibit any fixed ratio 

 between the lime and magnesia, and carbonate of iron 

 occasionally forms a real constituent of the mineral. Such 

 facts could be reconciled with the laws of Dalton only 

 by supposing the interference of a new law, that of 

 Isomorphism. 



It is now sufficiently established that certain elements 

 are closely related to each other, so that they can, as it 

 were, step into each other's places without apparently 

 altering the form of the compound molecules, or the 

 shape of the crystals which they constitute. The car- 

 bonates of iron, calcium, and magnesium, are nearly 

 identical in their crystalline forms, hence they may 

 crystallize together in harmony, producing mixed minerals 

 of considerable complexity, which nevertheless perfectly 

 verify the laws of equivalent proportions. This principle 

 of isomorphism once established, not only explains what 

 was formerly a stumbling-block, but gives most valuable 

 aid to chemists in deciding upon the real constitution of 

 new salts, since those compounds of isomorphous elements 

 which have identical crystalline forms must possess cor- 

 responding chemical formulae. 



We may always expect that from time to time new and 

 extraordinary phenomena will be discovered, and will lead 

 to new views of the laws of nature. The recent observa- 

 tion, for instance, that the resistance of a bar of selenium 

 to a current of electricity is affected in an extraordinary 



