AN UNDISCOVERED GAS. -253 



myself, and a set of systematic diftusious of nitrogen was carried out. 

 After 30 rounds, corresponding to 180 diffusions, the density of the 

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

 diflnsed most slowly had there been any difference in rate was identical 

 witli that of the most quickly diffusing portion; i. e., with that of the 

 portion which passed first through the j)orous plug. This attempt, 

 therefore, was unsuccessful, but it was worth carrying 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 exjjeri- 

 ments 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. 



Tlie apparatus used for diffusion had a capacity of about 2 liters. It 

 was filled with helium, and the operation of diffusion was carried 

 through 30 times. There were 6 reservoirs, each full of gas, and each 

 was separated into two by diffusion. To the heavier portion of one lot 

 the lighter portion of the next was added, and in this manner all G reser- 

 voirs were successively passed through the diffusion apparatus. This 

 process was carried out 30 times, each of the G 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 rediffusion, 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 quan- 

 tity 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 

 lightest x>ortion of these gases was 1.98, and after other 15 diffusions 

 the density of the lightest portion had not decreas^ed. The end had 

 been reached; it was not possible to obtain a lighter portion by diffu- 

 sion. The density of the main body of this gas is therefore 1.98, and 

 its refractivity, air being taken as unity, is 0.1215. The spectrum of 

 this portion does not differ in any respect from the usual spectrum 

 of helium. 



As rediffusion does not alter the density or the refractivity of this 

 gas, it is right to suppose that either one definite element has now 

 been isolated, or that if there are more elements than one present they 

 possess the same or very nearly the same density and refractivity. 

 There may be a group of elements, say three, like iron, cobalt, and 

 nickel ; but there is no proof that this idea is correct, and the simplicity 

 of the spectrum would be an argument against such a supposition. 

 This substance, forming by far the larger part of the whole amount of 

 gas, must, in the present state of our knowledge, be regarded as pure 

 helium. 



