TRANSACTIO>'S ©F SECTION B. 597 



or thereabouts. Tke difEculty was increased by the curious behaviour, which we 

 have often had occasion to confirm, that helium possesses a rate of dilfusion too 

 rapid for its density. Thus, the density of the lightest portion of the diffused gas, 

 calculated from its rate of diffusion, was 1"874 ; but this corresponds to a real 

 density of about 2'0. After our paper, giving an account of these experiments, 

 had been published, a German investigator, Herr A. Hagenbach, repeated our 

 work and confirmed ©ur results. 



The two samples of gas of dilTerent density differ also in other properties. 

 Different transparent substances differ in the rate at which they allow light to pass 

 through them. Thus, light travels through water at a much slower rate than 

 through air, and at a slower rate through air than through hydrogen. Now Lord 

 liayleigh found that helium offers less opposition to the passage of light than any 

 other substance does, and the heavier of the two portions into which helium had 

 been split ofi'ered more opposition than the lighter portion. And the retardation 

 of the light, unlike what has usually been observed, was nearly proportional to the 

 densities of the samples. The spectrum of these two samples did not differ in the 

 minutest particular ; therefore it did not appear quite out of the question to hazard 

 the speculation that the process of diflusion was instrumental, not necessarily in 

 separating two kinds of gas from each other, but actually in removing light 

 molecules of the same kind from heavy molecules. This idea is not new. It had 

 been advanced by Prof. Schiitzenberger (whose recent death all chemists have to 

 deplore), and later, by Mr. Crookes, that what we term the atxjmic weight of an 

 element is a mean ; that when we say that the atomic weight of oxygen is 1 6, 

 we merely state that the average atomic weight is 16; and it is not inconceivable 

 that a certain number of molecules have a weight somewhat higher than 32, while 

 a certain number have a lower weight. 



We therefore thought it necessary to test this question by direct experiment 

 with some known gas ; and we chose nitrogen, as a good material with which to 

 test the point. A much larger and more convenient apparatus for diffusing gases 

 was built by Mr. Travers and myself, and a set of systematic diffusions of nitrogen 

 was carried out. After thirty rounds, corresponding to 180 diffusions, the density 

 of the nitrogen was unaltered, and that of the portion which should have diffused 

 most slowly, had there been any difference in rate, was identical with that of the 

 most quickly diflusing portion — i p., with that of the portion which passed first 

 through the porous plug. This att^empt, therefore, was unsuccessful ; but it was 

 worth caiTying out, for it is now certain that it is not possible to separate a gas of 

 undoubted chemical unity into portions of different density by diffusion. And 

 these experiments rendered it exceedingly improbable that the difference in density 

 of the two fractions of helium was due to separation of light molecules of helium 

 from heavy molecules. 



The apparatus used for diffusion had a capacity of about two litres. It was 

 filled with helium, and the operation of diffusion was carried through thirty times. 

 There were six reservoirs, each full of gas, and each was separated into Wo by 

 diffusion. To the heavier portion of one lot, the lighter portion of the next was 

 added, and in this manner all six reservoirs were successively passed through the 

 diffusion apparatus. This process was carried out thirty times, each of the six 

 reservoirs having had its gas diffused each time, thus involving 180 diffusions. 

 After this process, the density of the more quickly diffusing gas was reduced to 

 2-02, while that of the less quickly diffusing had increased to 2-27. The light 

 portion on re-diffusion hardly altered in density, while the heavier portion, when 

 divided into three portions by diffusion, showed a considerable difference in density 

 between the first third and the last third. A similar set of operations was 

 carried out with a fresh quantity of helium, in order to accumulate enough gas to 

 obtain a sufficient quantity for a second series of diffusions. The more quickly 

 diffusing portions of both gases were mixed and rediffused. The density of the 

 lightestportion of these gases was 1'98 ; and after other 15 diffusions, the density 

 of the lightest portion had not decreased. The end had been reached ; it was not 

 possible to obtain a lighter portion by diffusion. The density of the main body 

 of this gas is therefore 1*98 ; and its refractivity, air being taken as unity, is 



