124 Day and Allen — Isomorphism and Thermal 



a pair of curves, I, taken from the microcline measurements, 

 is reproduced in the adjoining figure (fig. 10), and appears 

 to show such an absorption clearly, extending from 1135° 

 to 1275°. The dotted line shows the course of the curve 

 without the absorption, as inferred from the glass curve. 

 The same figure contains two other curves (II, III), similarly 

 treated, which were made upon fresh charges of the same 

 material, but with different rates of heating. It will be 

 noticed that the absorption begins to be noticeable at a slightly 

 lower temperature if the heating is slower. 



This peculiar behavior shown by compounds which melt to 

 form hyperviscous liquids seems not to have been observed 

 before and to contain features of more than ordinary interest. 

 Here are evidently crystalline substances which not only can 

 exist for considerable periods of time at temperatures far above 

 their melting temperatures, but which melt with extreme slow- 

 ness in the lower portion of this range of instability. It would 

 certainly be no exaggeration to say that the albite with which 

 we worked would require some weeks to reach the amorphous 

 state if maintained at a constant temperature of 1125°. 



An interesting question arises here as to the state of the 

 crystalline material at temperatures above its melting point. 

 It is easily conceivable that the crystals are merely superheated 

 without loss of any of their properties as solids, and that they 

 thus present an analogy to superheated liquids. In the trans- 

 formation (Umwandlung) of a solid crystalline substance into 

 another crystal form, such superheating has long been known. 

 The change is dependent upon temperature and pressure like 

 ordinary fusion, but it is possible to pass the transformation 

 temperature in either direction. This must be due to the 

 unfavorable opportunity for molecular motion which solids 

 afford, and the latter should differ in no essential particular 

 from ultra viscosity. 



On the other hand, it does not seem a violation of any 

 known principle to conceive cases of unstable equilibrium in 

 which the molecules of a liquid are oriented as in a crystal. 

 Maxwell's demons might arrange them much like a school of 

 fish and there is no apparent reason why the fluidity should be 

 destroyed thereby. Were such an arrangement one of mini- 

 mum potential, the mass would be a liquid crystal. In the 

 supposed case such a substance would possess a melting point 

 dependent upon the temperature and pressure, above which 

 Maxwell's definition* of a true solid — that its viscosity be 

 infinite — would no longer obtain, although deorientation might 

 not become apparent, in the face of extreme viscosity, for a 



* Maxwell's Scientific Papers, vol. ii, p. 620. 



