408 PROCEEDINGS OF THE AMERICAN ACADEMY. 



In other words, we have no right to imagine that the space is " free " or 

 that there is a hard particle in the centre ; the shell is as essential an 

 attribute of the atom as the centre. But how are we to account for heat 

 vibration, if the atom is supposed to fill the whole space ? This question 

 is important; but before answering we must consider some of the con- 

 sequences of this form of compression. 



Let us imagine two highly elastic spheres ; for example, two very thin- 

 walled india-rubber balls filled with gas. Imagine these to be drawn 

 together by a powerful attraction resideut throughout themselves. When 

 they come in contact, each will compress the other and evolve heat in 

 the process. They will remain bound together and distorted, unless some 

 force separates them. If the shell of an atom is elastic and compressible, 

 it is only reasonable to suppose that the interior is also. In that case 

 the whole substance of both of two combining atoms will suffer distortion 

 from the mutual attraction of every part of their substance ; and the con- 

 centration of those constituents in each atom which cause the affinity will 

 thus be increased in the half nearest the other atom. The supposition that 

 the affinity comes from within will cause here an essential divergence from 

 the actual conditions in two balls filled with gas, in which the gas is distrib- 

 uted equally throughout. As a consequence, the opposite half which is 

 not combined will lose some of it attractive constituents, aud should then 

 have less tendency to unite with the new substances than it had before 

 its union with the first atom. This plausible influence agrees with the 

 well-known facts of " false equilibrium " and the nascent state ; in fact, 

 it would account in general for the permanence of slightly stable 

 compounds. 



By the process of hypothetical reasoning given above, one concludes 

 that the whole substance of the atom may be elastic. In that case heat 

 vibration might consist simply in alternate condensation and rarefaction 

 of the medium within the shell, started by the momentum of impact. This 

 would continue indefinitely, unless the vibration were imparted to other 

 substances possessing less. Such internal rarefaction and condensation 

 might well tend to distend the atom if any portion of the atom were held 

 by another. 



Thus, it is evident that there is no difficultv in imagining internal 

 vibration in an atom which is packed on all sides closely with other 

 atoms, or in explaining the mechanism of the thermal expansion of solids 

 and liquids upon that basis. The chief reason for imagining a small 

 hard particle with a large free space around it is therefore removed. 



Two other reasons for retaining the conception of the old atom may be 



