44 ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS. 



There is no circulation in these viscous melts and nothing to assist 

 in distributing the heat uniformly. The melting point is, therefore, 

 not marked by a constant temperature but by the point of greatest 

 inclination of the tangent to the curve, with a limit of error which 

 increases with increasing viscosity. With Ab 3 Ani it was barely dis- 

 cernible, and with Ab 4 An x all trace of the heat of fusion was lost.* 

 Slow heating or rapid heating merely acts to change the general incli- 

 nation of the curve but not to emphasize the absorption of heat. 



By wav of conveying a concrete impression it may be added that 

 Ab 3 Ani just above its melting temperature resists the introduction of 

 a stout platinum wire (1.5 mm. diameter) unless the cold wire is thrust 

 in very quickly and vigorously. If the wire is first allowed to become 

 hot in the furnace, it will give way itself instead. No acceleration of 

 the melting process tending to sharpen the break in the curve appears 

 to be possible without the introduction of new substances or new con- 

 ditions (water vapor under pressure for example) which would take 

 the experiment outside the definition of a "dry melt." We have 

 undertaken some preliminary experiments in these directions, but 

 they belong to another phase of the subject. 



A number of efforts were made to locate the melting temperature of 

 Ab 3 Ani, which are given in the list below. Although two days were 

 required to crystallize each charge of the material sufficiently for a 

 determination, the recorded numbers possess but little significance, as 

 will be clear from the foregoing. 



Ab 3 An x (Plates XIV, XV, XVI). 



FIRST PREPARATION. 



Approximate melting temperature, 1340 . 



* Only a small portion of the charge could be crystallized. The relatively small 

 heat of fusion of the crystallized portion was, therefore, superposed upon the 

 larger specific heat of the glass. This, together with the effect of the viscosity, 

 destroyed all record of the melting. 



