CHEMICAL ELEMENTS AND ATOMS — URBAIN 219 



utmost confidence since he has found the way of making them ex- 

 emplify all tlie laws necessary for the sustaining of our science. 

 AVe can declare ourselves satisfied. The classification of Mendeleeff 

 has now solid foundations. It can await new facts and feel equal 

 to them. To have seen it thus in the past come out the stronger 

 after each fray, we feel all the more confident in its fate. Glory 

 to Mendeleeff, and glory to you, his most ardent disciple, glory to 

 the founder of comparative chemistry. 



It is good to have found a scientific base for this study of chemical 

 analogies. Strict science did not require it, but the demon of curi- 

 osity which is within us desired it. Between you and me, I believe 

 we are always like children and ask "Why? " 



'■ Why " do chemical analogies exist ? Take the case of electrons. 

 When half of the universe is made of electrons they have to assume 

 many responsibilities, especially when the other half shrivels up, so 

 well protected from experiment that it refuses to play a part in the 

 phenomena which, except for radioactivity, are just beyond our ken. 

 " Electron " answers all " whys " of physics and chemistry. They 

 have much to do. 



Maybe we have only translated into a new language, very poetic 

 and full of imagery, the questions which our demon of curiosity poses 

 for us in a very prosaic language. We, indeed, express to-day the 

 same idea in saying either dysprosium and holmium are very closely 

 related, or the atoms of dysprosium and of holmium have both sev- 

 eral circlets of exterior electrons. If our demons give the second 

 statement in the form of a question we may be sure the first will 

 be the answer. 



We have here, then, as a mathematician would say, reciprocal 

 propositions. Let us hope for their proof. Meanwhile we say that 

 two distinct atoms with identically the same circlets of exterior 

 electrons behave chemically and physically in the same manner. 

 Thus we interpret isotopes. It little matters whether they are 

 radioactive or not, the idea of isotopy has become general. It was 

 inevitable. 



Among the nonradioactive elements, isotopy was, during its period 

 of incubation, only an elegant interpretation of a new spectroscopy 

 due to the inventive genius of J. J. Thomson. He dispersed in a 

 Crookes tube, the canal rays — that is, the positive rays — hj means 

 of a double field, electric and magnetic. He thus obtained parabolas 

 due to atomic trajectories which were so great in number and of 

 such quality that in order to interpret them it was necessar}^ to con- 

 sider our elements as mixtures impossible to separate by any other 

 means. 



The experiments of Aston upon neon brought a real explanation 

 of the variation of density of neon in fractional osmosis. 



