376 Professor G. D. Liveing [May 15, 



always producing waves in the ether at the rate of many billions a 

 second. We are sure that they have a great deal of eneTgy, and if 

 they cannot move far they must have very rapid vibratory motion. 

 It is reasonable to suppose that the parts of each molecule swing 

 backwards and forwards through, or about, the centre of mass of the 

 molecule. The average distance to which the parts swing will give 

 the average dimensions of the molecule. 



Dalton fancied that he had proved that the atoms of the chemical 

 elements must be spherical, because there was no assignable cause 

 why they should be longer in one dimension than another. I rather 

 invert the argument. I see no reason why the excursions of the parts 

 of a molecule from the centre of mass should be equal in every direc- 

 tion. I assume, as the most general case, that these excursions are 

 unequal in different directions, and since the movements must be 

 symmetrical with reference to the centre of mass, they will in general 

 be included within an ellipsoid, of which the centre is the centre of 

 mass. 



Here I may, perhaps, guard against a misconception. Chemists 

 are familiar with the notion of complex molecules, and most of us 

 figure to ourselves a molecule of common salt as consisting of an atom 

 of sodium and an atom of chlorine held together by some sort of force, 

 and it may be imagined that these atoms are the parts of the molecules 

 which I have in mind. That, however, is not my notion. I am 

 paradoxical enough to disbelieve altogether in the existence of either 

 sodium or chlorine in common salt. Were my audience a less philo- 

 sophical one, I could imagine the retort on many a lip — " Why, you 

 can get sodium and chlorine out of it, and you can make it out of 

 sodium and chlorine." But, no ; you cannot get either sodium or 

 chlorine out of it WT.thout first adding something which seems to me of 

 the essence of the matter. You can get neither sodium nor chlorine 

 from it without adding energy. Nor can you make salt out of those 

 elements without subtracting energy. My point is that the energy is 

 of the essence of the molecule. Each kind of molecule has its own 

 kind of motion ; and in this I think most physicists will agree with 

 me. The chemists will agree with me in thinking that all the mole- 

 cules of the same element or compound are alike in mass, and in the 

 space they occupy at a given temperature and pressure. The only 

 further assumption I have to make is that the form of the ellipsoid, 

 the relative length of its axes, is on tlie average the same for all the 

 molecules of the same substance. This implies that the distances 

 of the excursions of the parts of the molecule depend upon its con- 

 stitution, and are, on the average, the same in similarly constituted 

 molecules under similar circumstances. 



I have now come to the end of my postulates. I hope they are 

 such as you will readily concede. I want you to conceive of each 

 molecule that its parts are in extremely rapid vibration, so that it 

 occupies a larger space than it would occupy if its parts were all at 

 rest ; and that the excursions of the parts about the centre of mass 



