Page:Popular Science Monthly Volume 47.djvu/540

526 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 nitrogen 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 conclusive 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 impossible 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 nitrogen—show the same peculiarity. On the other hand, a definite melting point, a definite boiling point, a definite critical temperature 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 interest 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