IO ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS. 



tropical jungle which will almost certainly lead to further successful 

 explorations. 



Meantime the results reached are of great importance. The 

 melting points of the triclinic feldspars have been determined with 

 an accuracy never before attained in determinations at such tem- 

 peratures. Any future correction of them must be of very trifling 

 amount. These points, considered with reference to composition, 

 and the very fine series of specific gravity determinations on chemi- 

 cally pure feldspars, seem to settle beyond question the isomorphism 

 of the plagioclases. The first cogent arguments for this isomorphism 

 were given by Sartorius von Walthershausen in 1853, but the more 

 thorough investigation of Tscherrnak in 1864 has properly connected 

 the theory with his name. Nevertheless, some of the ablest investi- 

 gators have been unconvinced that the isomorhpism was complete, 

 and I confess to surprise that the proof is so irrefragable as Messrs. 

 Day and Allen have made it. 



The study of the feldspars and sodium tetraborate (dehydrated 

 borax) have confirmed results of Professor Lagorio, which are thus 

 summarized by Mr. Teall (op. cit., p. 397) : 



Silicate solutions differ from aqueous solutions in the readiness with which they 

 form amorphous glass when cooled rapidly. This appears to be connected with 

 the fact that they may be readily overcooled, and that when in this state they are 

 highly viscous, so that a rapid approach of the molecules is prevented. The melt- 

 ing point of glass is lower than that of the same substance in a crystalline condi- 

 tion. A glass, therefore, results from the solidification of an overcooled liquid. 



Messrs. Day and Allen show that crystallization can be brought 

 about at very different degrees of overcooling or at very different 

 temperatures, so that the solidifying temperature of crystals out of 

 undercooled liquids is not a physical constant, while solidification to 

 the amorphous state almost or quite eludes determination by the 

 means found adequate to fix the melting points of crystals.* 



* The properties of amorphous substances are very perplexing. It is well known 

 that some physicists class glasses at any temperatures as liquids, and there is no 

 question that it is hard to draw the line between them and liquids. On the other 

 hand, Mr. Spring has recently shown that mere deformation of crystalline metals 

 at ordinary temperatures changes their densities and electrical potentials, so that 

 mere derangement of crystalline particles, without any absorption of energy com- 

 parable with that accompanying true fusion, suffices to impart to lead, silver, bis- 

 muth, etc., properties analogous to those of glasses. The whole subject demands 

 fuller investigation which, to be successful, must harmonize thermal, electrical, and 

 mechanical phenomena. 



