ARGON, A NEW CONSTITUENT OF TIIK ATMOSPHERE. 7 



Tile method of ino.st uiiiversiil .•i|)[ilic:itioii by wliicli to test whether u gas is 

 piii'e t)i- a mixture of (■()mi)()iieiits of dilfeiciit densities is that of diffusion. By this 

 means Graham succeeded in effecting a ]>ai(ial sep.-.iation of tlie nitrogen and oxy- 

 gen of the air, in spite of the conii)ai;ttively small difference of densities. If tlie 

 atmo.sphere contains an unknown gas of anything like tlie density supjjosed, it 

 should be possible to prove the fact by opei'atious conducted upon air which had 

 undergone atmolysis. Tf, for example, the parts least disposed to penetrate porous 

 walls were i-etained, the " nitrogen " derived from it by the usual processes should 

 be heavier than that derived in like manner from unprepared air. This experi- 

 ment, although in view tVom the first, was not executed until a later stage of the 

 inquiry (§ 6), when results were obtained sufiicieut of themselves to prove that the 

 atraosphei'e contains a previously unknown gas. 



But although the method of diffusion was capa))le of deciding the main, or at 

 any I'ate the first, question, it held out no prospect of isolating the new constituent 

 of the atmosphere, and we therefore turned our attention in the first instance to 

 the consideration of methods more strictly chemical. And hei'e the question 

 forced itself upon ns as to what really was the evidence in favor of the prevalent 

 doctrine that the inert residue from air, after withdi'awal of oxygen, water, and 

 cai'bonic anhydride, is all of one kind. 



The identification of " [)hlogisticated aii-" with the constituent of nitric acid is 

 due to Cavendish, whose method consisted in operating \vith electric sparks upon a 

 short column of gas confined with potash over raei'cury at the upper end of an in- 

 verted U-tube.^ This tube (M) was only about j\ inch in diameter, and the column of 

 gas was usually about one inch in length. After describing some preliminary trials, 

 Cavendish pi'oceeds : "I introduced into the tube a little soap-lees (potash), and 

 then let up some dephlogisticated (oxygen) and common air, mixed in the above- 

 mentioned proportions, wdiich, I'ising to the top of the tube (M), divided the soaji- - 

 lees into its two legs. As fast as the air was diminished by the electric spark, I 

 continued adding more of the same kind, till no further diminution took place ; 

 after which a little pure dephlogisticated aii', and after that a little common air, were 

 added, in order to see whether the cessation of diminution was not owing to some 

 imperfection in the piopoi'tion of the two kinds of aii' to each other ; but without 

 effect. The soap-lees being then poured out of the tube, and separated from the 

 quicksilver, seemed to be perfectly neutralized, and they did not at all discolor 

 paper tinged with the juice of blue flowers. Being evaporated to dryness, they left 

 a small C[uantity of salt, which was evidently nitre, as appeared by the manner in 

 which papei', impregnated with a solution of it, bui-ned." 



' Experiments on Air, Philosophical Transactions, vol. LXXV, p. 372, 1785. 



