408 Sir W. Crookes. On the Position of Helium, 



atmosphere, have probably been misled by the close proximity of the 

 brilliant yellow line of krypton to the helium line. 



On the assumption of the truth of Dr. Johnstone Stoney's hypo- 

 thesis that gases of a higher density than ammonia will be found in 

 our atmosphere, it is by no means improbable that a gas lighter than 

 nitrogen will also be found in air. We have already spent several 

 months in preparation for a search for it, and will be able to state 

 ere long whether the supposition is well founded. 



"On the Position of Helium, Argon, and Krypton in the 

 Scheme of Elements." By SIR WILLIAM CROOKES, F.R.S. 

 Eeceived and Read June 9, 1898. 



It has been found difficult to give the elements argon and helium 

 (and I think the same difficulty will exist in respect to the gas 

 krypton) their proper place in the scheme of arrangement of the 

 elements which we owe to the ingenuity and scientific acumen 

 of JSTewlands, Mendeleef and others. Some years ago, carrying a 

 little further Professor Emerson Reynold's idea of representing the 

 scheme of elements by a zigzag line, I thought of projecting a scheme 

 in three dimensional space, and exhibited at one of the meetings of 

 the Chemical Society* a model illustrating my views. Since that 

 time, I have re-arranged the positions then assigned to some of the 

 less known elements in accordance with later atomic weight deter- 

 minations, and thereby made the curve more symmetrical. 



Many of the elemental facts can be well explained by supposing the 

 space projection of the scheme of elements to be a spiral. This curve 

 is, however, inadmissible, inasmuch as the curve has to pass through 

 a point neutral as to electricity and chemical energy twice in each 

 cycle. We must therefore adopt some other figure. A figure-of-eight 

 will foreshorten into a zigzag as well as a spiral, and it fulfils every 

 condition of the problem. Such a figure will result from three very 

 simple simultaneous motions. First, an oscillation to and fro 

 (suppose east and west) ; secondly, an oscillation at right angles to 

 the former (suppose north and south), and thirdly, a motion at right 

 angles to these two (suppose downwards), which, in it simplest form, 

 would be with unvarying velocity. 



I take any arbitrary and convenient figure-of-eight, without refer- 

 ence to its exact nature ; I divide each of the loops into eight equal 

 parts, and then drop from these points ordinates corresponding to the 

 atomic weights of the first cycle of elements. I have here a model 

 representing this figure projected in space; in it the elements are 



* Presidential Address to the Chemical Society, March 28, 1888. 



