208 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1925 



professional habits, and the purism of academic methods, main- 

 tained lon<r a<^o the difference between these two sciences, a dis- 

 tinction originally drawn by an arbitrary classification which 

 divided into compartments the continuum of the sciences. 



When our knowled<>e was less, it was easy to <!:ive the chemical 

 elements a simple and a])parently decisive delinition as Lavoisier 

 did. To respect his definition, at least formally, it would be neces- 

 sary to-day to distinguish between two methods of analysis, that 

 of chemistry and that of physics. To the contingencies involved 

 in the very nature of things there are then added artificial limita- 

 tions. When we form a scientific ideal we should weed out these 

 conditions as much as possible, no matter why they were imposed. 

 Lavoisier, in order to define a simple body, had to clear away 

 the vapors of phlogiston and renounce the ideas of a then decrepit 

 atomism. AVhat would he have thought of our present ideas and 

 theories? Would the numerous presumptuous accumulations in 

 favor of the atomic structure suffice to prove its reality to him? He, 

 and all the encyclopcedists of the eighteenth century, would have 

 reproached us, perhaps, concerning this imagined existence of atoms, 

 this act of faith. 



The atoms, so far as any direct manifestation to our senses is 

 concerned, will aways elude us because of their extreme minuteness. 

 We can not ever hope to see them because we laiow beforehand that 

 our microscopes, and even our ultramicroscopes, however i)owerful 

 they may be, have a very limited magnification which would always 

 be insufficient. In this direction the matter is definitely settled. 

 We must be reconciled to the certainty of never seeing them and 

 be contented to know them through their effects. In taking these 

 evidences we are like a judge before the accused, and we must fear 

 errors of judgment. The multiplicity of the accusing evidence is 

 doubtless very impressive. Is the question clear enough so that 

 we can give a certain judgment? 



We consider effects since they are our sole evidence. Then we 

 postulate the atom that we may the better interpret these facts — in 

 no way making the atom a necessary objective reality. Jean Perrin, 

 who has gone deepest in this matter, has appealed to probabilities. 

 How did he prove the reality of molecules in his now famous 

 memoir? Doubtless he has applied very precise exponential laws 

 resulting from direct experiment. But it was not with molecules 

 themselves that he experimented, but with colloidal aggregates which 

 he called composite molecules. Without doubt he may diminish 

 more and more the diameter of these aggregates, but there remains 

 a gap between the smallest of them and the molecules which he may 

 never close. When he coinits the molecules in a vertical cylinder of 



