662 THE PRINCIPLES OF SCIENCE. [chap. 



ignorant of more complex phenomena involved in their 

 application. Newton's comprehension of geometrical optics 

 was sufficient to explain all the ordinary refractions and 

 reflections of light. The simple laws of the bending of 

 rays apply to all rays, whatever the character of the 

 undulations composing them. Newton suspected the 

 existence of other classes of phenomena when he spoke of 

 rays as having sides; hut it remained for later experi- 

 mentalists to show that light is a transverse undulation 

 like the bending of a rod or cord. 



Dalton's atomic theory is doubtless true of all chemical 

 compounds, and the essence of it is that the same com- 

 pound will always be found to contain the same elements 

 in the same definite proportions. Pure calcium carbonate 

 contains 48 parts by weight of oxygen to 40 of calcium 

 and 12 of carbon. But when careful analyses were made 

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

 was unquestionably the same mineral, judging by its 

 crystalline form and physical properties, would give varying 

 proportions of its components, and would sometimes contain 

 unusual elements which yet could not be set doAvn as 

 mere impurities. Dolomite, for instance, is a compound of 

 the carbonates of magnesia and lime, but specimens from 

 different places do not exhibit any fixed ratio between the 

 lime and matrnesia. 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 established that certain elements are related to 

 each other, so that they can, as it were, step into each other's 

 places without apparently altering the shapes of the crystals 

 which they constitute. The carbonates of iron, calcium, 

 and magnesium, are nearly identical in their crystalline 

 forms, hence they may crystallise together in harmony, 

 producing mixed minerals of considerable complexity, 

 which nevertheless perfectly verify the laws of equivalent 

 proportions. This principle of isomorphism once esta- 

 blished, not only explains what was formerly a stumbling- 

 block, but gives valuable aid to chemists in deciding vipon 

 the constitution of new salts, since compounds of isomor- 

 phous elements which have identical cr3'staUine forms 

 must possess corresponding chemical formulae. 



We may expect that from time to time extraordinary 



