561 Stoney — Cause of Double Lines in Spectra. 



the ratio of the specific heat at constant pressure to the specific heat at constant 

 volume, is nearly the same for all these gases, and that its value is 1'408. 



From these data the kinetic theory of gases enables us to gain our first insight 

 as to what is going on within the molecules. These experiments, when inter- 

 preted by its help, show that only 0'612 of the energy in the more perfect gases is 

 accounted for by the motions of the molecules as they dart about amongst one 

 another like missiles, and that the remaining 0'388 of the whole energy is the 

 energy of events that are going on within the molecules themselves. 



We learn from electrical, thermal, and spectroscopic observations that energy 

 can pass from the molecules of a gas to the aether; and we know tliat when a gas 

 warms its containing vessel or expands against pressure, external work is done by 

 it upon ponderable matter. Now it is very important to observe at the threshold 

 of our inquiry that these are the only tvays in which any energy whatever is 

 expended by a gas. Retarding forces of one kind or other arise in all the motions 

 with which we are most familiar on the earth, because the motions we are 

 accustomed to take notice'of are molar motions of the irrotational type, pursued 

 under such conditions that part of the molar energy is constantly leaking down 

 into subsidiary molecular activities. When, however, we get to the bottom of the 

 series of irrotational motions, beneath which there are none underlying, no such 

 degradation of energy is possible. Accordingly there is absolutely no loss of 

 energy in the encounters between molecules ; neither is there a trace of anything 

 like friction or viscosity between the different parts of a molecule to impede any 

 events that may be going on witliin it during its flight between one encounter and 

 the next. All its internal motions are even more free from any such interference 

 than are the motions of the planets, which are affected, in a minute degree, both by 

 meteors and by tidal actions. 



Very striking information about these internal events is furnished by the 

 spectroscope, which reveals to us the fact that they are such as to occasion definite 

 undulatory changes in the surrounding aether. Each gas is in fact found to emit an 

 interrupted spectrum, consisting of separated lines ; of which the positions, intensi- 

 ties, and general appearance are characteristic of the molecules from which they 

 emanate. We thus become aware of the fact that each gaseous molecule, and as a 

 consequence each chemical atom, is an elaborate system within which highly com- 

 plex changes of a periodic character are perpetually taking place. 



The object of the present communication is to try to cross the threshold of an 

 inquiry as to what these molecular events are. 



In this investigation we shall have to treat of periods of time too small to be 

 conveniently spoken of as fractions of a second. And, fortunately, the nature of 

 the subject indicates the best way of dealing with them. This is by making use 

 of the velocity of light and other electro-magnetic waves in the open aether, which 



