440 PRINCIPLES OP CHEMISTRY 



i.e.y taking 0=16, the atomic weight of hydrogen is equal not to 1 bufc 

 to a greater number somewhere between 1-002 and 1-008 or mean- 



be expressed in whole numbers with reference to hydrogen, or if they at least proved to 

 be commensurable with one another, then it could be affirmed with confidence that the 

 elements, with all their diversity, were formed of one material condensed or grouped in 

 various manners into the stable, and, under known conditions, undecomposable groups 

 which we call the atoms of the elements. At first it was supposed that all the elements 

 were nothing else but condensed hydrogen, but when it appeared that the atomic weights 

 of the elements could not be expressed in whole numbers in relation to hydrogen, 

 it was still possible to imagine the existence of a certain material from which both hydro- 

 gen and all the other elements were formed. If it should transpire that four atoms of this 

 material form an atom of hydrogen, then the atom of chlorine would present itself as 

 consisting of 142 atoms of .this substance, the weight of whose atom would be equal to 

 0'25. But in this case the atoms of all the elements should be "expressed in whole 

 numbers with respect to the weight of the atom of this original material. Let us sup- 

 pose that the atomic weight of this material is equal to unity, then all the atomic weights 

 should be expressible in whole numbers relatively to this unit. Thus the atom of one ele- 

 ment, let us suppose, would weigh m, and of another n, but, as both m and n must be 

 whole numbers, it follows that the atomic weights of all the elements would be commen- 

 surable. But it is sufficient to glance over the results obtained by Stas, and to be 

 assured of their accuracy, especially for silver, in order to entirely destroy, or at least 

 strongly undermine, this attractive hypothesis. We must therefore refuse our assent to the 

 doctrine of the building up from a single substance of the elements knqwn to us. This 

 hypothesis is not supported either by any known transformation (for one element has never 

 been converted into another element), or by the commensurability of the atomic weights 

 of the elements. Although the hypothesis of the formation of all the elements from a 

 single substance (for which Crookes has suggested the name protyle) is most attractive 

 in its comprehensiveness, it can neither be denied nor accepted for want of sufficient data. 

 Marignac endeavoured, however, to overcome Stas's conclusions as to the incommensu- 

 rability oft the atomic weights by supposing that in his, as in the determinations of all 

 other observers, there were unperceived errors which were quite independent of the mode 

 of observation for example, silver nitrate might be supposed to be an unstable substance 

 which changes, under the heatings, evaporations, and other processes' to which it is sub- 

 jected in the reactions for the determination of the combining weight of silver. It might 

 be supposed, for instance, that silver nitrate contains some impurity which cannot be 

 removed by any means ; it might also be supposed that a portion of the elements of the 

 nitric acid are disengaged in the evaporation of the solution of silver nitrate (owing to the 

 decomposing action of water), and in its fusion, and that we have not to deal with normal 

 silver nitrate, but with a slightly basic salt, or perhaps an excess of nitric acid which 

 cannot be removed from the salt. In this case the observed combining weight will nt 

 refer to an actually definite chemical compound, but to some mixture for which there 

 does not exist any perfectly exact combining relations. Marignac upholds this proposition 

 by the fact that the conclusions of Stas and other observers respecting the combining 

 weights determined with the greatest exactitude very nearly agree with the proposition 

 of the commensurability of the atomic weights for example, the combining weight of 

 silver was shown to be equal to 107'98, so that it only differs by 0'08 from the whole 

 number 108, which is generally accepted for silver. The combining weight of iodine 

 proved to be equal to 126'85 that is, it differs from 127 by 0'15. The combining weights 

 of sodium, nitrogen, bromine, chlorine, and lithium are still nearer to the whole or round 

 numbers which are generally accepted. But Marignac's proposition will hardly bear 

 criticism. Indeed if we express the combining weights of the elements determined by 

 Stas in relation to hydrogen, the approximation of these weights to whole numbers 

 disappears, because one part of hydrogen in reality does not combine with 16 parts of 

 oxygen, but with 15'92 parts, and therefore we shall obtain, taking H = 1, not the above- 

 cited figures, but for silver 107-38, for bromine 79*55, magnitudes which are still further 



