322 



Prof. W. Ramsay and Mr. M. W. Travers. 



the jar in parallel and a spark gap interposed the blue spectrum of 

 argon was equally distinct in both tubes ; and, more important still, 

 there was no trace of any unknown line. It appears, therefore, that 

 helium contains no unknown gas, nor is it possible to separate it by- 

 diffusion into any two kinds of gas ; all that can be said is that; 

 most minera]s which evolve helium on heating also evolve argon in 

 small quantity. This accounts for the difference in density observed 

 in different samples of helium ; and in one instance, viz., malacone, 

 the amount of argon evolved on heating the mineral, though small, 

 was much in excess of the helium, so far as could be judged by the 

 spectrum. 



In the light of the experiments of which an account has here been 

 given, it is necessary to reconsider the deduction drawn by Professors 

 Runge and Paschen from the complex nature of the spectrum of 

 helium as regards its complex nature. Sir Norman Lockyer has 

 already pronounced in favour of the supposition that helium is a 

 mixture, chiefly on the ground that in the spectra of certain stars 

 some, but not all, of the helium lines are observable. It appears to 

 us that this may well be accounted for by the hypothesis that the 

 differences of temperature and pressure in the stars might produce 

 variations in the spectrum of helium. If a jar and spark gap be 

 interposed while observing the visible spectrum of helium, a pro- 

 found alteration is to be noticed. The yellow line D 3 is to be seen 

 near the electrodes, and is faint in the capillary portion of the tube, 

 and one of the red lines disappears. The change is not as remark- 

 able as in the case of argon, but is quite distinct and characteristic. 

 Then, as before remarked, the green line becomes relatively stronger 

 at low pressures, so that the light evolved in the tube is no longer 

 the usual brilliant yellow, but dull greenish-purple. Is it not Jikely 

 that the conditions obtaining in the stars may account for the absence 

 of some of the lines ordinarily visible ? 



If the hypothesis of Runge and Paschen is correct, then the two 

 gases to which they attribute the complex spectrum of helium must 

 have nearly the same density. It has already been shown that by 

 means of the apparatus used for the fractional diffusion of gases it is 

 possible to effect a fair separation of the constituents of air after a 

 few rounds. If the supposed constituents of helium differ in den- 

 sity in as high a proportion as 14 to 16, it is certain that some separa- 

 tion would have been effected. As there has been no such separation, 

 the legitimate inference is that the density of the two supposed con- 

 stituents does not differ by so great an amount, or that their exist- 

 ence is imaginary. It appears to us that too little is known regarding 

 the nature of the vibrations which cause spectra to make it legitimate 

 to theorise on the subject. It is surely conceivable that an atom may 

 possess such a structure as to render it capable of propagating two 



