1 so BRIDGMAN. 



the eflFect of the shape of the atoms, we have in our hands at least a 

 possibihty of explaining the complicated facts of polymorphism. As a 

 matter of scientific economy, we are bound to push as far as we can 

 the possibilities of the next stage of refinement. The discussion will 

 in part take the form of showing in detail how some of the unexpected 

 effects which we have found may be exhibited by aggregates of build- 

 ing blocks with definite shapes. 



The crystal is supposed to be composed of units, atoms or molecules 

 as the case may be, which remain the same in different polymorphic 

 forms. Polymorphism is to be regarded in its most general aspect as 

 due to regrouping of these units in different arrangements. This 

 does not rule out at all the possibility of such special groupings as are 

 considered in the association theories in which larger related groups 

 may be distinguished. Each one of the units is to be thought of as 

 terminated by rigid boundaries, that is, each unit has a shape as defi- 

 nitely as a brick has shape. Furthermore, at different localities on 

 the surface of the units there are localized centers of force (attractive 

 usually) so that two units, if free, will tend to come together with a 

 definite orientation. A crystal is to be regarded as a system in which 

 a compromise has been affected between the arrangement which the 

 units would take in virtue of the action of the localized centers of 

 force, the arrangement into which the units would be urged by the 

 external pressure or the mean internal pressure so as to occupy the 

 smallest possible volume, and the chaotic disarray which temperature 

 agitation tends to produce. 



This conception of an atom or molecule as a hunk of matter of 

 definite shape and with localized centers of force is no doubt crude 

 from certain points of view. We require a more detailed picture 

 to account for the scattering of a particles, for example. But it does 

 seem to contain enough of the essentials of the situation to make it a 

 suggestive tool of thought in dealing with polymorphic changes. 

 Doubtless a more valid picture of the atom is as a field of force. But 

 in the last analysis this comes down to much the same thing as saying 

 that the atom has shape. All that we mean when we say that any 

 object of our experience has shape or boundaries is that as we approach 

 the object the force with which it acts on us changes at a certain stage 

 very rapidly into an intense repulsion. Our idea of shape is only a 

 qualitative one, depending on how fast the repulsive force increases. 

 It is worthy of remark that if we regard the atom as essentially a field 

 of force we shall find that in order to account for the observed facts 

 this field of force must become very rapidly a repulsion beyond a 



