270 CRYSTALLIZA.TION. 



always radiating, wliicli means producing waves in the aether at the 

 rate of many billions in a second. We are sure that they have a great 

 deal of energy, and, if they can not move far, they mnst have very 

 rapid vibratory motions. It is reasonable to suppose that the parts of 

 each molecule swing, backwards and forwards, through, or about, the 

 center of mass of the molecule. Tlie average distances to which the 

 parts swing will determine the average dimensions of tlie molecule, 

 the average space it occupies. 



Dalton fancied 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 his 

 argument. I see no reason why the excursions of the i^arts of a mole- 

 cule from tlie centre of mass should be equal in all directions, and 

 therefore assume, as the most general case, that these excursions are 

 unequal in different directions. And, since tlie movements must be 

 symmetrical with reference to the centre of mass of the molecule, they 

 will in general be included within an ellipsoid, of which the center is 

 the centre of mass. 



Here I may [)erhaps guard against a misconception. We 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 one 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 para- 

 doxical enough to disbelieve altogether in the existence of either 

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

 phical one I could imagine I heard the retort from 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 can not get either sodium or chlorine 

 out of common salt without first adding something which seems to me 

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

 from it without a(hling energy; nor can you make it out of these ele- 

 ments without sul)tracting energy. My point is that energy is of the 

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

 and in this I think most physicists will agree with me. Chemists will 

 agree with me in thinking that all the molecules of the same element, 

 or compound, are alike in mass, and in the space they occupy at a 

 given temperature and pressure. The only remaining assumption I 

 make is that the form of the ellipsoid — the relative lengths of its axes — 

 is on the average the same for all the molecule sof the same substance. 

 This implies that the distances of the excursions of the parts of the 

 molecule depend on its constitution, and are, on the average, the same 

 in similarly constituted molecules under similar circumstances. 



I have 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 as 

 having its parts in extremely rapid vibration, so that it occupies a 

 larger space than it would occupy if its parts were at rest; and that 



