Physical and Chemical Behavior of Solids. 249 



This hypothesis can be made use of only when the deforma- 

 tion is permanent ; in other words, it is inapplicable (and 

 unnecessary) so long as the material is within its elastic limit. 

 It applies primarily to crystalline aggregates, but this state- 

 ment does not imply that it accounts completely for the 

 behavior of systems composed of a single crystal or of similar 

 crystals similarly oriented — in short, of systems where the 

 vectorial crystal forces are more or less dominant — when 

 exposed to the action of external forces. Indeed, it in no 

 wise precludes the use of other explanations to account, for 

 instance, for the presence and occurrence of cleavage and 

 gliding planes in crystals, which may well be a different phe- 

 nomenon, associated or not with the phenomena discussed in 

 the foregoing pages. Yet so far as the writers are aware, 

 the explanation of gliding planes commonly advanced and 

 accepted by crystallographers and geologists does not conflict 

 with that which follows from the viewpoint of the present 

 paper in any point which can be decided by direct experi- 

 mental evidence at the present time. 



It may be of service, in rendering the conception more con- 

 crete, to present a brief outline of a possible mode of action of 

 unequal pressure on crystalline solids. We will suppose that 

 the ultimate crystal particle is the real molecule of the sub- 

 stance ; this molecule is in general, if we may say so, a 

 polymer of the simplest chemical formula which expresses the 

 relative composition of the substance, its mean degree of 

 polymerization depending on the external conditions of tem- 

 perature and pressure. In other words, dissociation of the 

 complex increases with increasing temperature (and decreasing 

 pressure), until at a certain point, so much of the new phase 

 (still in general a polymer, though less complex than that 

 stable at lower temperatures and higher pressures) is formed 

 that a transformation occurs ; this point is either a transition 

 point or a melting point, according as the new phase is crystal- 

 line or fluid.* 



* Similarly, for transformations of the liquid phase into another phase, 

 either liquid or gaseous, the gas phase is usually considered to be made up of 

 molecules polymerized very slightly, if at all. The above viewpoint would 

 seem to be quite generally accepted in the case of the substance water, at 

 least, steam being supposed to be (H 2 0)„, liquid water (H 2 0) 2n , and ice 

 (H 2 0) 3)l or (HO) 4 », n being a small integer ; for this particular substance, 

 indeed, it would be difficult to imagine a way of accounting for the maximum 

 density, except by a hypothesis involving the presence in the liquid phase 

 of more than one molecular species. Further evidence in favor pf this view- 

 point is afforded by facts such as the possibility of heating certain solids 

 above the melting point, and the behavior of certain substances on melting, 

 e. g., antimony if heated only slightly above its melting point, but still to 

 all appearances completely melted, and then cooled, may crystallize just as 

 if the original crystal framework had not been destroyed. 



