Properties of the Feldspars. 105 



ing (10 min. per 1°) gave a temperature a few degrees lower, 

 but such variations as could be applied within the period of a 

 working day did not suffice, under the most favorable condi- 

 tions, to change this temperature materially. The first evi- 

 dence of molecular mobility in borax glass, shown in the 

 sticking together of the finest particles (sintering), and the 

 first traces of crystallization and release of latent heat, appeared 

 consistently at about 490-500°. Still a third phenomenon 

 attracted our attention to this temperature. On every occa- 

 sion when borax glass was heated rapidly, either powdered or 

 in the solid block, a slight but persistent absorption of heat 

 appeared in this same region and continued over some 20°, 

 after which the original rate of heating returned. We were 

 entirely unable to explain an absorption of heat in an amor- 

 phous substance under these conditions except by assuming an 

 actual change of state to exist between amorphous glass and 

 its melt, in which case the absorbed heat would reappear some- 

 where upon the corresponding cooling curve, — which it failed 

 to do. We then reasoned that any assumed change in the 

 molecular structure which would account for an absorption of 

 heat would also be likely to cause an interruption in the con- 

 tinuity of the curve of electrical conductivity, and the relative 

 conductivity was determined throughout this region ; but no 

 such interruption appeared. 



Finally the matter was abandoned. The evidence did not 

 appear sufficient to establish any discontinuity in the cooling 

 curve of the glass, so long as no crystallization took place. 



When these relations had been clearly established, we turned 

 again to the feldspars. 



It became clear very early in the investigation that only 

 artificially prepared and chemically pure specimens would be 

 adequate for our purpose. Each of the end members of the 

 series, anorthite and albite, as found in nature, is alvyays inter- 

 mixed with some quantity of the other, while the intermediate 

 members generally contain iron and potash, and all are liable 

 to inclusions. 



There was nothing new in this plan : Fouque and Levy* had 

 demonstrated the possibility of making pure feldspars by 

 chemical synthesis and had studied their optical properties 

 some years ago. We undertook to prepare much larger 

 quantities than they (200 grams), and to make a careful study 

 of their heating and cooling curves under atmospheric pres- 

 sure, the conditions under which anorthite and the plagio- 

 clases crystallize, the relations between the amorphous and 

 crystalline forms, the sintering of crystalline and vitreous pow- 

 ders, in short, their entire thermal behavior, as we had done 

 with the borax. At the same time it was our purpose to make 



* Synthase des Mineraux et des Eockes, loc. cit. 



