366 Dr. H. Debus on the Genesis of 



the compounds formed of these elements. The sum of the 

 weights of the atoms in a molecule is his molecular weight. 



Avogadro determines the molecular weights directly from 

 the specific gravities of the gases. The molecular weight is, 

 if I may say so, the end of Dalton's and the beginning of Avo- 

 gadro' s work. About the constituents of the molecules, the 

 atoms, Avogadro is absolutely silent. He knows only mole- 

 cules and parts of molecules. Whether there is or is not a 

 limit to the division of a molecule he does not say. Avogadro 

 treats the subject like a theoretic mechanician, who considers 

 his particles divisible or not, according to the nature of his 

 problem. The unit of Dalton is the indivisible atom, the unit 

 of Avogadro the divisible molecule (D. 70). 



It is worthy of notice that Avogadro is not conscious of 

 these differences between Dalton and himself. He criticises 

 Dalton's work as if he and Dalton were engaged with the 

 solution of the same problems, and calling attention to the 

 agreement of some of his results with those of Dalton, he 

 says : — " This agreement is in favour of our hypothesis, 

 which is essentially nothing else but the system of Dalton 

 improved by a sure expedient* to which we have been led by 

 the facts discovered by Gray-Lussac " (0. no. 8, p. 22). 



Avogadro could only identify his system with Dalton's on the 

 assumption that both systems rested on the same principle. 

 This principle is the hypothesis M/S = 0. As Avogadro received 

 his information about Dalton' s work from Thomson, it follows 

 that he put the same interpretation on Thomson's words as I 

 have done, and as the latter published his book in the year 

 1807, and Avogadro his essay in 1811, it appears highly 

 probable that Avogadro received the first intimation of the 

 hypothesis M/S = C from Dalton. The great merit of 

 Avogadro is founded on his proposal to consider the mole- 

 cules of the elementary gases divisible into two or more parts, 

 because thereby the hypothesis M/S = C became of practical 

 importance. 



But are the hypothetical molecular weights of Avogadro 

 the real molecular weights? This question could only be 

 answered by the synthesis of the molecules from the con- 

 stituent atoms. The sum of their weights is the molecular 

 weight. Therefore, logically considered, the atomic weights 

 must precede the molecular weights. The determination of 

 the atomic weights was, accordingly, the task of Dalton and 

 his generation. This is the real reason why the hypothesis 

 M/S = C received no attention in Dalton's and Avogadro's 

 time and was soon forgotten. Fifty years later, after 

 * Tlie division of molecules. 



