INERT CONSTITUENTS OF THE ATMOSPHERE. 595 



the very electro-positive elements of the lithium group ; and that, in con- 

 sequence of their lack of electric polarity, and their inactivity, they 

 form, in a certain sense, a connecting link between the two. It is 

 curious, too, to notice that iodine, xenon, caesium and barium form 

 the ends of their respective columns. It is, of course, not impossible 

 that other elements may be discovered, possessing similar properties, 

 and yet higher atomic weights than these; but as yet there is no clue 

 to guide us where to search for them. 



It is difficult, owing to the impossibility of effecting a complete 

 separation of the inactive elements from each other, to do more than 

 hazard a guess as to their relative amount in air. As they are easily 

 separated from the other constituents of air, there is no doubt as to their 

 total amount; air contains 0.937 parts of argon and its companions 

 by volume in 100 parts. Perhaps the table below may be taken as 

 affording some indication of their relative amounts. Air contains by 

 volume : 



0.937 parts of argon per hundred. 



One or two parts of neon per hundred thousand. 



One or two parts of helium per million. 



About one part of krypton per million. 



About one part of xenon per twenty million. 



It is of course not impossible that xenon may contain an even 

 smaller proportion of a still heavier gas ; but it is unlikely. Sea- water 

 sometimes contains a grain of gold per ton; that is one part in 

 15,180,000; a grain of xenon is contained in about four hundred- 

 weights of air. 



The problems suggested by the periodic table are by no means solved 

 by the discovery of these aerial gases; but something has been done 

 to throw light upon one obscure comer of the field. The gap between 

 the electro-positive and the electro-negative elements has been bridged. 



