THE KINETIC THEORY OF GASES. 285 



the vertical columns, their position should be for helium before lithium, 

 and for argon before potassium, thus: 



Hydrogen. . 

 Fluorine . . . 



. 1 

 . 19 



Helium . 



? 



. i 



Lithium 



Sodium 



. 7 

 . 23 



Chlorine . . . 



. 35.5 



Argon . . 



. 10 



Potassium. 



. 39 



Manganese. 



. 55 



1 Iron 



Cobalt. 

 [Nickel .'. 



56] 



■ 58 1 



59 



Copper 



. 63.5 



Bromine 



. 80 



? 





Rubidium . 



. 85 



Now, we find the difference between the atomic weights of hydrogen 

 and chlorine to be 34.5; and between lithium and potassium to be 32; 

 also between argon and helium to be 36. These numbers are roughly 

 of the same order of magnitude. It is, therefore, not unreasonable to 

 suspect the existence of an undiscovered element with atomic weight 

 between 19 and 23, as well as of others occupying the other unfilled 

 positions in the argon group. 



It is no easy matter to hunt the earth through for an unknown ele- 

 ment. The question is, where to look. And some clew is necessary to 

 guide the inquiry. At first it was thought that minerals similar to 

 those from which lielium had been obtained might possibly yield the 

 new element; and experiments were made, for months at a time, to test 

 the gases obtainable from almost every known mineral, but in vain, so 

 far as a new element was concerned. They resulted in the discovery 

 of many new sources of helium, but the spectrum of the gas in each 

 case exhibited no unknown lines. A new method of attack was then 

 organized. It might be that the so-called helium was really a mixture 

 of elements, and not a pure element. Now, an effective method of sepa- 

 rating from each other two gases of different molecular weights, and 

 hence of different densities, is the process of diffusion. From observa- 

 tions of the late Professor Graham, of University College, London, 

 subsequently maste'r of the mint, it appears that lighter gases, with 

 rapidly moving molecules, will pass through a porous diaphragm, such 

 as the material of a clay pipe, more rapidly than a heavier gas, with its 

 more slowly moving molecules. An attempt was therefore made to 

 ascertain whether any heavier gas could be thus separated from the 

 helium obtained from minerals. The experiments involved an enormous 

 amount of labor, but in the end no gas other than a trace of argon could 

 be detected. It appeared, therefore, vain to attempt to discover a new 

 gas in minerals; and the justice of Dr. Stoney's hypothesis was next 

 tested. It was, of course, not out of the question that the sought-for 

 gas might exhibit some powers of combination, and that it might have 

 been absorbed, along with the nitrogen of the air, by the magnesium 

 over which the gas had been sent at a red heat, in order to absorb and 

 remove the nitrogen. The compound of magnesium with nitrogen is 

 very readily decomposed by water; the products are ammonia and 

 hydroxide of magnesium. A large quantity of this magnesium nitride 



