heating the tube. The conclusion therefore follows that no helium is present in our atmosphere. It is, besides, exceedingly improbable that helium should be present. For, as shown by Dr. Johnstone Stoney, the rate of motion of a molecule of hydrogen is so rapid that, on finding its way to the confines of the atmosphere, it would escape, and travel through space until it found a planet of sufficient attractive force to hold it. The same is true of helium. Were helium present in the atmosphere, it would ultimately leave us for the sun, or for some planet of much greater mass than the earth. This conception of Dr. Stoney’s tallies with the observation that the moon, a planet of small mass, is devoid of an atmosphere, and that the sun, a body with a mass 300,000 times as great as the earth, shows in its chromosphere the spectra both of hydrogen and of helium with great brilliancy. It is now of interest to inquire what are the properties of argon and how it is related to other elements. [25] _Proc. Roy. Soc._ vol. liii. p. 134. [26] _Proc. Roy. Soc._ vol. lix. p. 68. _Phil. Trans._ vol. clxxxvi. p. 227. [27] _Compt. rend._ vol. cxxi. p. 394. CHAPTER VI THE PROPERTIES OF ARGON The density of a gas is one of its most characteristic and important properties. Avogadro’s law, which postulates that equal volumes of gases, at equal temperature and pressure, contain equal numbers of molecules, renders it possible to compare the weights of the molecules by determining the relative weights of the gases. Thus, as the ratio between the densities of nitrogen and oxygen is 7 to 8, a single molecule of nitrogen, the smallest portion which can exist in freedom, uncombined with other elements, is ⅞ths of the weight of a single molecule of oxygen. Hence a determination of the density of argon leads directly to a knowledge of the relative weight of a single molecule of this gas. But with what should the density of argon be compared? What gas must serve as the standard of density? To answer this question, it is necessary to give a short sketch of the development of chemical theory regarding the atomic weights of elements and their relative volumes. Dalton proposed to adopt as the unit of atomic weight the weight of the lightest atom, namely, that of hydrogen. Taking, for example, water as one substance containing hydrogen, its percentage composition by weight is-- Hydrogen 11·11 per cent Oxygen 88·88 "