THE KINETIC THEORY OF GASES. 283 



that of the planets that, even at the exceedingly high temperature 

 which its atmosphere possesses, it is impossible for any known gas to 

 remove itself from the neighborhood of the luminary. 



We must now take leave of Dr. Stoney's fascinating hypotheses for 

 a time, and consider the recent discoveries of gaseous constituents of 

 our atmosphere. 



After the -discovery of argon as a constituent of air in 1894, one of 

 the discoverers, acting an advice given him by Professor Miers, was so 

 fortunate as to isolate helium, a gas contained in certain rare minerals, 

 the best known of which bears the name of clcveite. Helium had 

 previously been detected in the chromosphere, the colored atmosphere 

 of the sun, by M. Janssen, the well-kuown French astronomer; and its 

 name was suggested by Messrs. Fraukland and Lockyer, in 18(58, to 

 characterize the brilliant yellow line by which its presence in the sun 

 is revealed. Neither of these elements has been combined with others, 

 although it is possible that each exists in combination with one or more 

 of the elements contained in the minerals from which helium can be 

 obtained by heating, for it has been found that small quantities of argon, 

 along with considerable quantities of helium, are evolved from such 

 minerals. Again, both of these elements possess one curious property, 

 which they share with gaseous mercury alone, so far as is known, among 

 all elements. That is technically called the ratio between their specific 

 heats at constant pressure and at constant volume. It would be difficult 

 here to set forth the reasoning by which it is deduced that inasmuch as 

 the ratio for these gases is If to 1 between specific heat at constant 

 pressure and at constant volume, the molecules of these elements, 

 unlike those of oxygen and hydrogen and the other commoner gases, 

 but like those of mercury gas, consist not of agglomerations of two or 

 more atoms, but of single atoms. These characteristics at once estab- 

 lish a connection between the two elements helium and argon, and 

 differentiate them in kind from all other gaseous elemeuts. 



ISTow, taking the density of hydrogen as unity, that of helium is 

 very nearly 2, and that of argon 20. And one of the conclusions which 

 follows from the Kinetic theory of gases is that equal volumes of gases 

 contain equal numbers of molecules. Thus the fact that helium is 

 twice as heavy as hydrogen carries with it the conclusion that a mole- 

 cule of helium is twice as heavy as a molecule of hydrogen, whatever 

 the absolute weight of the latter may be. 



Now, it can be demonstrated that there is a strong probability in 

 favor of the assumption that a molecule of hydogen consists of two 

 atoms, inseparable from each other unless by combination with some 

 other element. And if a molecule of helium consisting of one atom is 

 twice as heavy as a molecule of hydrogen consisting of two, then it 

 follows that an atom of helium is four times as heavy as an atom of 

 hydrogen.; in other words, the atomic weight of helium is 4, that of 



