6 Ramsay, The Newly Discovered Elements. 



sists of two atoms, while that of mercury is identical with 

 its atom. And this agrees with the conclusion drawn from 

 the ratio between the specific heats of mercury-gas, namely, 

 that the molecule of mercury-gas is mono-atomic ; that it 

 is a smooth elastic sphere. 



The ratio between the specific heats of hydrogen, of 

 oxygen and of nitrogen, unlike that of mercury-gas, is 1*4, 

 approximately. This is also the case with carbon mon- 

 oxide, CO, and with nitric oxide, NO, both of which gases 

 are probably di-atomic. The conclusion would follow by 

 analogy that the molecules of the three elementary gases 

 are also di-atomic, and that their formulae should be 

 H^, O^ and Nl Gases with three atoms in their molecules, 

 however, do not show such regularity as these ; the ratio 

 varies between i'2o for carbon disulphide, to i"3i for sul- 

 phuretted hydrogen. But generally, it has been found 

 that the greater the number of atoms in the molecule of a 

 gas, the lower the ratio of the specific heats. The 

 physicist, Boltzmann, has devised a theorem to account 

 for this fact. He imagines a molecule consisting of one 

 atom to possess what he terms three degrees of freedom ; 

 that is, it may move in space in all conceivable directions, 

 like a fly in the air ; but all these directions may be 

 referred to motion in three directions at right angles to 

 each other. A molecule resembling a smooth, elastic 

 sphere, however, is not capable of acquiring by collision 

 with other molecules, or with the walls of the vessel in 

 which it is contained, any motion of the nature of rotation 

 about its axis. On the other hand, a molecule consisting 

 of two atoms possesses, like the former, three degrees of 

 freedom, and, in addition, other two ; that is, it is free to 

 rotate in two planes, each parallel to the line joining the 

 centres of its atoms ; but it is not free to rotate in a plane 

 at right angles to that line. It thus possesses five degrees 



