SOME CONTEMPORARY .IDr.lXCIS IX I'lD'SlCS—rr 481 



which jjast's |jrcsi>iU ovi-ii iti small proportions in a (!is<!iarm-lul)f 

 roiiid Ih' (k'tiTtwl ami idontihed. The first trials wore naturally 

 made ii|x>n disrharjje-tulK"s containing the commoner gases, which 

 as it hapfHMis nearly all consist of one kind of atom (or molecule) 

 apiece — oxygen, nitrogen, hydrogen, carbon dioxide, carbon mon- 

 oxide. This retarded the great discovery. But when neon, a gas of 

 presununl atomic weight 20.2, was introduced into the tube in l'.tl2, 

 Thomson observed two pencils, of atoms of masses about 20 and 22, 

 resfx,'Ctively, where he h,i<l cxpi-ctod to see but one consisting of atoms 

 of mass about 20.2." 



This observation was not immcfliatciy interpreted as we now 

 interpret it. The mysterious pencil might have consisted <jf molecules 

 of CO-j of mass 44 bearing a double charge, or of molecules of a hitherto 

 unknown compountl XcHj. These possibilities were tested by ap- 

 propriate ex|X'riments and discarded, and then for a time the gas of 

 atomic mass 22 was apparently regarded as a new element distinct 

 from neon and fortuitously mixed with it. 



F. \V. Aston undertook the attempt to separate the two gases, but 

 they were so entirely alike in their properties that no success what- 

 ever was attained by fractional distillation and little by diffusion. 

 This was Aston's entr>- into this field, and in a celebrated series of 

 researches, soon interrupted by the war but resumed after six years 

 and still continuing, he associated his name forever with the analysis 

 of elements into the different kinds of atoms of which they consist. 



Of the improvements which Aston made in the method of measuring 

 the masses of charged particles, as of the details of Thomson's original 

 method and of Dempster's method, it i^hardly necessary to si)eak; 

 for they have Ix^en admirably described, with reproductions of photo- 

 graphs, in several recent books. '^ The problem of generating ions 

 of the elements to be analyzed became progressively harder to solve. 

 The elements gaseous at room-temp)eraturc were easily investigated, 

 and those of which a high vapor density could Ix? produced either of 

 the element or of one of its compounds, without overheating the 

 tul)e, were also tractable; but when these elements had all been tested 

 the resistance to further advance became formidable. Ions of the 

 thirteen elements lately analyzed were formed as anode rays: that is, 

 they were charged atoms expelled from the anode of a discharge-tube 



" Neon l)y virtue of its well-known chemical inertness has no "combining" weight, 

 but its average molecular weight was determined from its density by Watson, using 

 .\vogadro's principle, as 20.200. Thomson's earliest e.xperimenls were not delicate 

 enough to distinguish whether the atoms in the former of the two [x;ncils were of 

 mass 20.0 or of mass 20.2, but the difference between either and 22 was unmistakable. 



" Notably in .Aston's own lxK)k Isotopes and in .-Xndrade's The Structure of the Atom. 



