February 7, 1895 J 



NA TURE 



JJO 



are also without action at a red heat. Platinum black does 

 not absorb it, nor does platinum sponge, and wet oxidising 

 and chlorinating agents, such as nitrohydrochloric acid, bro- 

 mine water, bromine and alkali, and hydrochloric acid and 

 potassium permanganate, are entirely without action. Kxpcri- 

 ments with (luorine are in contemplation, but the difficulty is 

 great ; and an attempt will be nude to produce a carbon arc 

 in the gas. Mixtures of sodium and silica and of sodium and 

 boracic anhydride are also without action, hence it appears to 

 resist attack by nascent silicon and by nascent boron. 



XVI. (ktieral Coiicltntoiis. 



It remains, finally, to discuss the probable nature of the gas, 

 or mixture of gKC'!, which we have succeeded in separating 

 from atmospheric air, and which we provisionally name argon. 



The presence of argon in the atmosphere is proved by many 

 lines of evidence. The high density of "atmospheric nitro- 

 gen," the lower density of nitrogen from ihemical source-, and 

 the uniformiiy in the density of samples of chemical nitrogen 

 prepared from difterent compounds, lead to the conclusion that 

 the cause of the anomaly is the presence of a heavy gas in air. 

 If that gas possess the density 20 compared with hydrogen, 

 "atmospheric" nitrogen should contain of it approximately i 

 per cent. This i«, in fact, found to he the case. Moreover, ns 

 nitrogen is removed from air by means of red-hot magnesium, 

 the density of the remaining gas rises proportionately to the 

 concentration of the heavier consiitueiit. 



Second. This gas has been concentrated in the atmosphere by 

 diffusion. It is true that it has not been freed from oxygen and 

 nitrogen by diffusion, but the process of diffusion increases, 

 relatively to nitrogen, the amount of argon in that portion 

 which does not pass through the porous walls. This has been 

 proved by its increase in density. 



Third. As the solubility of argon in water is relatively 

 high, it is to be expected that the density of the mixture of 

 argon and nitrogen, pumped out of water along with oxygen, 

 should, after the removal of the oxygen, be higher than that of 

 "atmospheric" nitrogen. Experiment has shown that the 

 density is considerably increased. 



Fourth. It is in the highest degree improbable that two pro- 

 cesses, so different from each other, should manufacture the 

 same product. The explanation is simple if it be granted that 

 these jirocesses merely eliminate nitrogen from ^ an "atmo- 

 spheric" mixture. Moreover, as argon is an element, or a 

 mixture of elements, its manufacture would mean its separation 

 from one of the substances emplo)ed. The gas which can be 

 removed from red-hot magne-ium in a vacuum has been found 

 to be wholly hydrogen. Nitrogen from chemical sources has 

 been practically all absorbed by magnesium, and also when 

 sparked in presence of oxygen ; hence argon cannot have 

 resulted from the decomposition of nitrogen. That it is not 

 produced from oxygen is sufficiently borne out by its preparation 

 by means of magnesium. 



Other arguments could be adduced, but the above are 

 sufficient to justify the conclusion that argon is present in the 

 atmospheie. 



The identity of the leading lines in the spectrum, the similar 

 solubility and the similar density, appear to prove the identity 

 of the argon prepared by both processes. 



.\rgon is an element, or a mixture of elements, for Clausius 

 has shown that if K be the energy of translatuiy motion of tlic 

 molecules of a gas, and II their whole kinetic energy, then 



K _ 3(C/-C,) 



C/> and C: denoting as usual the specific heat at constant 

 pressure and at constant volume respectively. Hence if, as for 

 mercury vapour and for aigon (§ .KIV.), the ratio of specific 

 heats C/ : Cz, be r-j, it follows that K = H, or that the whole 

 kinetic energy of the gas is accounted for by the translatory 

 motion of its molecules. In the case of mercury the absence of 

 interatomic energy is regarded as proof . f ihe monatomic 

 character of the vapour, and the conjlusion holds equally good 

 for argon. 



The only alternative is to suppose that if .nrgon molecules are 

 di- or polyatomic, the atc^ms .icquire no relative morion, even of 

 rotation, a conclusion excecdinj;ly improbable in iiself and one 

 postulating ihe sphericity of uch complex groups of atoms. 



Now a mon.iloinic gas can be only an element, or a mixture of 



NO. I 3 19, VOL. 5 1] 



elements ; and ht nee it follows that argon is not of a compound 

 nature. 



From Avogadro's law, the dersily of a gas is half its 

 molecular weight ; and as ihe density of argon is approximately 

 20, hence its irolecular weight must be 40. But its molecule is 

 idmtical with its at< m ; hi nee its atomic weight, or, if it lie a 

 mixture, the mean < f the aloinic weights of that mixture, taken 

 for the proportion in which they are present, must be 40. 



There is evidence both for and against the hypothesis that 

 argon 's a mixture : for, owing to Mr. Crcokes' observations of 

 the dual character of its spectrum ; against, because of Prof. 

 r)ls?ew-ki's sia'ement that it has a definite melting po-nt, a 

 definite boiling point, and a definite critical temperature and 

 pressure ; and because on compressing the gas in presrnce of its 

 liquid, pressure remains sensildy constant until all gas has con- 

 densed to liquid. The latter experiments are the wtll-known 

 criteria of a pure substance : the former is not known with cer- 

 tainty to be characteristic of a mix'ure. The conclusions which 

 follow are, however, so starti ng, that in our future experi- 

 mental work we shall indeavour to decide ihe question by othrr 

 means. 



For the present, however, the balince, of evidence seems to 



I point to simplicity. We have the e'^ore to discuss the relations 



to other elements of an element of atomic weight 40. We 'n- 



I { lined for long to the view that argon was possildy one or more 



than one of the elements which might be expected to follow 



' fluorine in the periodic classification of the elem'-nts — elemrnts 



which should have an atomic weight between 19, 'hat of fluorine, 



and 23, that of sodium. But this view is completely put out of 



court by the discovery of the monatomic nature of its molecules. 



The series of elements possessing atomic weights near 40 



are : — 



Chlorine ... ... ... ... 35'5 



Po'assium .. ... ... ... 39"! 



Calcium ... ... ... ... 400 



.Scandium ... ... ... ... 44'o 



There can be no doubt that potassium, calcium, and scan- 

 dium follow legitimately their predecessors in the vertical 

 columns, lithium, beryllium, and boron, and that they are in 

 almost certain relation with rubidium, strontium, and (but not so 

 certainly) yttrium. If argon be a single element, then there is 

 reason to doubt whether the periodic classification of the ele- 

 ments is complete; whether, in fact, elements may not exist 

 which cannot be fitted among those of which it is composed. 

 On the other hand, if argon be a mixture of two elements, they 

 might find place in the eighth group, one after chlorine and 

 one after bromine, .\ssuming 37 {the approximate mean 

 between the atomic weighlsof chlorine and poias-ium) to be 

 the atomic weight of the lighter element, and 40 the mean 

 atomic weight lound, and supposing that the second element 

 has an atomic weight between those of bromine. So, and rubi- 

 dium, 85 5, vi/. 82, Ihe mixture should consist of 933 per ccn'. 

 of the lighter, and 67 per cent of the heavier element. But it 

 appears improba'ile that such a high percentage as 67 of a 

 heavier element should have escaped detection during liquefac- 

 tion. 



If it be supposed that argon belongs to the eighth group, then 

 its properties would fit (airly well with what might be antici- 

 pated. For the series, which contains 



Si"', P^iiiar.'iv^ !^"t.V'/'> and CI ""■>■", 



might be expected to end with an element of monatomic mole- 

 cults, of no valency, i c\ incapable of forming a compound, or, 

 if forming one, being an nctad ; and it would form a possible 

 transition to po'assium, with its monovalence, on the o her 

 hand. Such concptions are, however, of a speculative 

 nature ; yet they may be perhaps excused, if they in any way 

 lead to experiments vihichtend to throw more light on the 

 anomalies of this lurious element. 



In conclusion, it need excite no astonishment that argon is 

 so indifferent to reagents. For mercury, although a monatomic 

 element, forms compounds which are by no means stable at a 

 high temperature in the gaseous state ; and attempts to pro- 

 duce compounds of argon m.iy be likened to attempts to cause 

 combination between mercury pas at 800° and other elements. 

 .\s for the physical condition of argon, that of a g.is, we possess 

 no knowledge why carbon, with its low atomic weight, 

 shoul I be a solid, while nitr 'gen is a gas, except in so far .is 

 we ascribe molecular complexify to the former and comparative 

 molecular simplicity to Ihe latter, .\rgon, with its compara- 



