486 BELL SYSTEM TECHNICAL JOURNAL 



procession of elements, comparisons with the oxygen atom become 

 fiifficult; hut adjacent elements can he intercompared. The eight 

 sorts of tin atoms lie next to the nine sorts of xenon atoms, the most 

 massive kind of tin agreeing closely in weight with the least massive 

 kind of xenon. W'luii atoms of gaseous xenon and molecules of a 

 volatile compound of tin are mixed together in the discharge-tube, the 

 beam of ions issuing through the cathode-perforation is resolved 

 into seventeen pencils; and the seventeen traces upon the plate are 

 so placed that the masses of the seventeen atoms cannot all be integer 

 multiples of a common unit of the order of I'l, the oxygen mass.'^ 

 bither the tin atoms or the xenon atoms deviate aiipreciabh' from 

 the rule, or possibly both do. 



So the common history of great sweeping discoveries in science 

 seems to repeat itself; the simplicity of the principle first announced 

 is gradually marred, its sharp lines become a triHe hazy and vague, 

 as experiments are multiplied and refined. Yet the principle does 

 not for that lose its character or its importance; the deviations of the 

 new group of values from integer numbers are small compared to those 

 of the old one, and promise to amplify the physical meaning of the rule 

 instead of restricting it. We should be less prepared to accept them, 

 were there not one of them at the very root of the system of elements; 

 for the mass of the hydrogen atom is not e.xactly the I'r, of the oxygen 

 mass which was taken for the fundamental unit mass of the system of 

 atoms, but is 1.008 IG of it. This seems embarrassing; the bricks of 

 which we intended to .say that the atomic structures are built turn out 

 to be smaller than the sample brick. But the embarrassment can be 

 removed: for it can be shown that of the mass of the hydrogen atom 

 is altogether electromagnetic, then the total mass of a group of such 

 atoms crowded closely together must he inferior to the sum of the 

 masses of the individual atoms when far apart. rheriforc. small 

 deviations from the rule of integer masses are to he anticii>aled, and 

 may be expected to serve as a most valuable controle of ]iroposed 

 models of atom-nuclei, wlu'ii the epoch of quantitative spatial models 

 arrives. This epoch nia\ lie distant ; or we may be upon the verge of it. 



We ha\e admitted, then, that the combining weight of an element, 

 being in general not its atomic mass hut the average of the masses 

 of several kinds of at<jms, and a weighted average at that, does not have 



"The c.\|)iTiiiieiU was piTfornicd with a liilx- containing the Ra-seous lonipoiind 

 tin tetranicthide (.SnClIjl^ and sonic .\enon from a previous e.vperiment. Kight 

 pencils of .SnClIa ions were oljserved, consisting of molecules comprising tin atoms of 

 the eight difTerent kinds; molecules containing tin atoms of mass 120 would have a 

 total mass of 1,?.S, and hence a pencil containing them would have fallen just midway 

 l)etween the pencils of xenon .itoms of masses IM and \Mt, respect ivel\': ,i( lu.diy it 

 fell distinctly off-centre. 



