February 7, 1895] 



NA TURE 



549 



Regarding it as estahlished that one or other of the gases 

 niu>t be a mixuire, containing, as I he ca^e m-ght he, an in^jre- 

 dient much heavier . r much lighter than oniinary n'trogen, we 

 had to cnnsidfr ihe relative proiiahiliiies of the vari .us possilile 

 inlerprelaiions. Except UDon the already discrediied h\po'.he<is 

 of dissociation, ii was difficult to see how the gas of chemical 

 origin could be a mixture. To suppose this would be to aimit 

 two kinds of ntric acifi, har<lly reconcilable wiih ihe work of 

 Slas and o'h'^rs upon the atomic weight of ihat subsrance. The 

 simplest explanation in many respects was to admit the exist- 

 ence of a yfcond ingreriient in air from which oxygeti, moisture, 

 and carbonic anhydride hid alrealy bee-n removed. The pro- 

 portional amount required was rol great. H the density of the 

 supposed gas were double that of nit'ogen ^ per cent, only by 

 volume would be needed ; or if the density were but half as 

 much agnin as that of ni:rogen, then I per cent, would still 

 suffice. But in accepting this explanation, even provisionally, 

 we had to face the improbabili'y [hat a gas surrounding us on all 

 sides, and present in enormous quandiies, could have remained 

 so long unsuspected. 



The method of most universal application by which to test 

 whether a gas is pure or a mixture of components of different 

 densities is that of dillusion. liy this means Graham succeeded 

 in effecting a partial separation of the nitrogen and oxygen of 

 the air, in spite of the comparatively snia 1 difference of 

 densities. If the atmosphere contain nn unknown gas of any- 

 thing 1 ke the density supposed, it should be possible to prove 

 the fact by o|ieraiions conducted upon air which had undergone 

 atmolysis. This experiment, although in view from the first, 

 was not executed until a later stage of the inqu ry (§6), when 

 results were obtained sufi'ioient of themselves to prove that the 

 Atmosphere contains a previously unknown gas. 



But although the method of diffusion was capable of decid- 

 ing the main, or at anv rate the first question, it held out no 

 prospect of isolating the new constituent of the atmo-|>here, and 

 we, therefore, turned our attention in the first instance to the 

 consideration of methods more strictly chemical. Aid here the 

 xjuestion fo'ced itself upon us as to what really was the evidence 

 in favour of the prevalent doctrine that the inert residue from 

 air after withdrawal of oxygen, water, and carbonic anhy- 

 dride, is all of one kind. 



The identification of " phlocisticaled air" with the con- 

 stituent of nitric acid is due to Cavend sh, whose metlud con- 

 sisted in operating with electric sparks upon a short column of 

 gas confi' cd with pjtash over mercury at the upper end of an 

 inverted (J tube.' 



Attempts to repeat Cavendish's experiments in Cavendish's 

 flianner have only increased the admiration with which we re- 

 gard this wondeiful investigation. Working on almost micro- 

 scopical quantities of mateiial, an^l by operations exienling 

 over days and weeks, he thus established one of the mo>t im- 

 portant facts in chemistry. And what is still more to the pur- 

 pose, he raises as distinctly as we could do, and to a certain 

 extent resolves, the question above suf^gested. The jiassageis 

 so important that it will be desirable to q'loie it at full length. 



" As far as the experiments hitherto published extend, we 

 scarcely know more of the phlogisiicated part of our atmo- 

 sphere, than that it is not diminished by lime-water, caustic 

 alkalies, or nitrous air ; that it is unfit to support fire, or mnin- 

 tain life in animals ; and that its specific graviiy is not much 

 less than that of common air : so that though the nitrous acid, 

 by being united to phlogiston, is converted into air possessed 

 of these p operties, and consequently, though it was reasonable 

 to suppose, that part at least of the phlogi-licated air of the 

 atmosphere consists of this acid united to phlogiston, yet it was 

 fairly to be doubted whether the whole is of this kind, or 

 whether there are not in reality many different substances 

 compounded together by us under the name of phlogi-ticated 

 air. I therefore made an experiment to determine whether the 

 whole ofa given portion of the phlogisiicated air of the atmo- 

 sphere could be reduced to nitrous acid, or wh« th-r theie was 

 not a part ofa different nature to the rest, which would refuse 

 to undergo lhat change. The foregoing exi-ciment^ indeed 

 in some measure decided this point, as much the grtatesl part 

 of the air let up into the tube lost its elasticity ; yet as some 

 remained unabsorbcd, it did not appear for certain wheihenhit 

 was of the same nature as the rest or not. Fir this purpose I 

 diminished a similar mixture of dephlogisticated an. I common 

 air, in the same manner as before, till it was reduced to a small 



I " ExperimcDts on Air," Phil. Trans, vol. I.xxv. p. 372,1785. 



NO. I319, VOL. 51] 



part of its original hulk. I then, in order to decompound as 



much as I could of the phlogisticated air which remained in the 

 lube, add. d some dephlogisticated air to i', and continued the 

 spark until no further diminution took place. Having by 

 these means condensed as much as I could of ihe phlogisiicated 

 air, 1 let up some solution of liver of sulphur to absorb the 

 dephlogisticated air : after which only a small lmt>ble of air 

 remained unabsorbed, which certainly was not more tn^n iljof 

 the bulk of the phlogisiicated air let up into the tube ; so itiat 

 if there is any part of ihe phlogisticated air of our atmo-phere 

 which differs from the rest, an I cannot be reduced to nitrous 

 acid, we may safely conclude lhat it is not more than il^th 

 part of the whole." 



Although Cavendish was satisfied with his result, and does 

 not decide whether the small residue was genuine, our ex- 

 periments about to be related render it not improbable that 

 his residue was really of a different kind from the main bulk of 

 the " phlogisticated air, " and contained the gas now called 

 argon. 



Cavendish gives data' from which it is possible to determine 

 the rate of ab_-.orption of the mixed gases in his experiment. 

 This was about I c.c. per hour, of which two-fifths would be 

 nitrogen. 



III. Methods of Causing Ficc Nitrogen to Combine. 



To eliminate nitrogen from air, in order to ascertain whether 

 any other gas could be detected, involves the use of some ab- 

 sorbent. Theelementswhich have been found to combine directly 

 with nitrOiien are: boron, silicon, titanium, lithium, strontium, 

 barium, magnesium, aluminium, mercury, and, under the 

 influence of an electric discharge, hydrogen in presence of acid, 

 and oxygen in presence of alkali Besides these, a mixture of 

 barium carbonate and carbon at a high temperature is known to 

 be tflfective. Of those tried, magnesium in the lorm of turnings 

 was found to le the best. When nitrogen is passed over 

 magnesium, heated in a tube of hard glass to bright redness, 

 combustion with incandescence begins at the end of the tube 

 through which the gas is introduced, and proceeds regularly 

 until all the metal has been converted into nitride. Between 

 7 and 8 litres of nitrogen can be absorbed in a single tube; 

 the nitride formed is a porous, dirty orange-coloured substance. 



XV. Early E.\-periments on Spariing JVitrogen with Oxygen 

 in preseitce of Alkali. 



In our earliest attempts to isolate the suspected gas by the 

 method of Cavendish, we used a RuhmkortV coil of medium 

 size aciuated by a battery of five Grove cells. The gases were 

 contained in a test-tube standing over a large quantity of weak 

 alkali, and the current was conveyed in wires insul ited by 

 (J-shaped glass tubes passing through the liquid round the 

 mouth of the test-tube. With the given battery and coil a 

 somewhat short spark or arc of about 5 mm. was found to be 

 more lavourat)le than a longer one. When the mixed gases 

 were in the right propoiliim the rate of absorption was about 

 30 c.c. per hour, or thirty times as last as Cavendish could work 

 with the electrical machine of his day. 



To lake an example, one experiment of this kind starled with 

 50 c.c. of air. To this oxygen was gradually added, until oxygen 

 being in excess, there was no perceptible contraction during an 

 hour's sparking. The remaining gas was then transferred at 

 the pneumatic trough to a small measuring vessel, sealed by 

 mercury, in which the volume was found to be 10 c.c. On 

 treatment with alkaline pyrogallate, the gas shrank to o 32 c.c. 

 That this small residue could not be nitrogen was argued from 

 the fact that it had withstood the prolonged action of ihe spark, 

 although mixed with oxygen in nearly the most favourable 

 prof>oition. 



The residue was then transferred to the test-lube with an 

 addition of another 50 c.c. of air, and the whole worked up 

 with oxygen as before. The residue was now 2'2 c.c, and, 

 after removal of oxygen, 076 c.c. 



Although it seemed almost impossible lhat these residues 

 could be either nitrogen or h\drogen, some anxiety was not un- 

 natural, seeing thai the final spaiking took place under some- 

 what abnormal conditions. The space was voty resiricltd, and 

 the temperature (and with it the proportion of aqueous vapour) 

 w.as unduly high. But any doubts lhat were fell upon this 

 score were removed by comparison experiments in which the 

 whole quantity of air operated on was very small. Thus, when 



' Phil. Tretfts. x'fl. Ixxviii. p. 271, 1788. 



