284 Lord Rayleigh and Prof. Ramsay. [Jan. 31, 



appears probable, argon be an element, or a mixture of elements, its 

 manufacture would mean its separation from one of the substances 

 employed. The gas which can be removed from red-hot magnesium 

 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 atmosphere. 



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

 bility and the similar density, appear to prove the identity of the 

 argon prepared by both processes. 



That argon is an element, or a mixture of elements, may be 

 inferred from the observations of XIV. For Clausius has shown 

 that if K be the energy of translatory motion of the molecules of a 

 gas, and H their whole kinetic energy, then 



K 



H " 20, 



C p and C v denoting as usual the specific heat at constant pressure and 

 at constant volume respectively. Hence if, as for mercury vapour 

 and for argon ( XIV), the ratio of specific heats C^, : C v be If, 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 of 

 the monatomic character of the vapour, and the conclusion holds 

 equally good for argon. 



The only alternative is to suppose that if argon molecules are di- 

 or polyatomic, the atoms acquire no relative motion, even of rotation, 

 a conclusion improbable in itself and one postulating the sphericity 

 of such complex groups of atoms. 



Now a monatomic gas can be only an element, or a mixture of 

 elements; and hence it follows that argon is not of a compound nature. 



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

 weight ; and as the density of argon is approximately 20, hence its 

 molecular weight must be 40. But its molecule is identical with its 

 atom ; hence its atomic weight, or, if it be a mixture, the mean of 

 the atomic 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 

 is a mixture : for, owing to Mr. Crookes' observations of the dual 

 character of its spectrum ; against, because of Professor Olszewski's 

 statement that it has a definite melting point, a definite boiling point, 



