526 TEE POPULAR SCIENCE MONTELT. 



other naturally occurring nitrogenous substances, most of which 

 are produced by the life-activity of micro-organisms ; and from 

 the natural substances all chemical compounds containing nitro- 

 gen are prepared. Considering, therefore, the identity of the 

 source, it seems improbable that the nitrogen of plants or animals 

 should contain argon, while that of inorganic chemical compounds 

 is without it. It is, however, possible that argon may enter the 

 plant in a manner quite different from nitrogen ; for it does not 

 follow that, because it is associated with nitrogen in the air, argon 

 must always play the part of an inseparable companion. 



Is argon an element, a mixture of elements, or a compound ? 

 While the evidence that it is a new substance is indisputable, the 

 facts thus far obtained do not warrant a final decision in regard 

 to its simplicity. There is no reason, however, to believe that it 

 is a compound, but, on the contrary, there is a piece of most con- 

 clusive evidence against this view. This evidence is the ratio of 

 its specific heats at constant pressure and at constant volume. 

 This has been carefully determined, and is found to be in exact 

 agreement with the value required by the mechanical theory of 

 heat for a monatomic gas that is, a gas whose molecules consist 

 of a single atom each. Such a state of things is obviously impos- 

 sible for a compound, which must have two atoms, at least, in 

 every molecule. It is also unusual in elementary gases, whose 

 molecules are in most cases diatomic, or of two atoms each. 

 Argon is therefore either an element or a mixture of elements 

 having structureless molecules. This evidence throws out of 

 court also the view, which has been repeatedly urged since the 

 first announcement of the discovery, that argon is an allotropic 

 form of nitrogen, consisting of triatomic nitrogen, and analogous 

 to ozone, which is triatomic oxygen. 



As to the question whether it is a single element or a mixture, 

 the argument for the mixture is based on the fact that it gives 

 two spectra. Though suggestive, this can not be looked on as 

 conclusive, for certain well-known elements hydrogen and nitro- 

 gen show the same peculiarity. On the other hand, a definite 

 melting point, a definite boiling point, a definite critical tempera- 

 ture and pressure, all of which argon possesses, are generally 

 accepted criteria of a pure substance. The evidence, therefore, is 

 largely in favor of the simple elementary character of argon. 



If subsequent investigation confirms this view, and argon 

 proves to be a single monatomic element, a question of great in- 

 terest is raised. For many years an accepted law of chemistry 

 has been expressed in the so-called periodic classification of the 

 elements. When the elements are arranged in the order of their 

 atomic weights, the series may be broken into a number of well- 

 defined periods, whose members show marked analogies to the 



