But how does the scientist proceed from the knowledge he has 

 thus gained, to an explanation of the periodic law? He has found 

 the atom complex; he has found it constituted of ions some positively 

 and some negatively charged. He now makes an assumption. He 

 assumes the positive charge to be a spherical shell surrounding the 

 negatively charged particles. Granting this assumption, the structure 

 of atoms containing various numbers of corpuscles can be mathe- 

 matically calculated; and the variations of structure with different 

 numbers of corpuscles in the atom are such as to explain in detail, 

 the Periodic Law, Group relations of elements. Series Relations, 

 Prout's Hypothesis, the Triads of Dobereiner, valency, chemical 

 action, the zero group, Radio activity, and unstable atoms. When it 

 is said that the variations of structure explain the variations of prop- 

 erties, it is meant that there are periodic recurrences of the same 

 structure where there are periodic recurrences of the same quality. 

 There are uniformities in structure corresponding to the uniformities 

 in property. 



It is evident that in this step in the theory, the scientist has changed 

 his method of procedure. Up to this step, he had established all upon 

 an experimental basis. Here he attempts to establish a step in his 

 theory upon other grounds. That the atom is a spherical shell of 

 positive electrification surrounding negative charges, he does not verify 

 experimentally. He assumes that it is true and according to the 

 established laws of physics, finds that the structural relations will be 

 such as to explain the chemical to a remarkable degree. The validity 

 of the assumption rests alone on its explaining power. The Atom may 

 not be formed as assumed; it may be but accidental that if it were, 

 all that correspondence between physical and chemical relations would 

 result, yet such chance occurrences are so anomalous in our experience, 

 that where we find such perfect correspondence we faithfully seek for 

 dependency. And when in an experimental chain, we are at a loss 

 for a link and we find one fitting perfectly, our experience has been 

 such that we take it as very probable that such a perfect fitting is not 

 accident, but that our assumed link is really existent and perhaps some 

 day to be experimentally revealed. 



THE THEORY OF INORGANIC EVOLUTION. 



The scientist's method of procedure is also well illustrated by the 

 theory of inorganic evolution presented in Duncan's "New 

 Knowledge." Some elements such as radium and uranium are found 

 to be actually evolving into other elements. Elements at different 



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