Properties of the Feldspars. 113 



From here on to the albite end of the series, viscosity becomes 

 very troublesome in restraining crystallization. The breaks 

 which mark the melting temperature on the heating curve of 

 Ab 3 An x are so slight as to make the determination difficult and 

 somewhat uncertain. It is not that temperature measure- 

 ment is less accurate here than elsewhere, for these tempera- 

 tures are more accessible than the melting point of anorthite, to 

 which reference has been made in this connection. These 

 ultra-viscous materials do not melt at a constant temperature 

 but over a considerable range of temperature, as we shall 

 undertake to show in some detail, with illustrations from pho- 

 tographs, in the discussion of albite. A glance at a series of 

 curves (fig. 7) plotted from our observations upon metallic silver 

 and the feldspars An, AbjAn 5 , Ab,An 2 and AbjAn, in such a 

 way as to bring their melting points together, will show clearly 

 the nature of this difficulty. The melting point of the metal 

 is sharp, but with anorthite a change in the character of the 

 phenomenon is noticeable. Its poor conductivity for heat and 

 its viscosity, which, though small compared with the other 

 feldspars, is very great compared with silver, have rounded 

 oft' the corners until a really constant temperature for a period 

 of a minute or more during the melting is nowhere to be 

 found. The nearest approach to a melting point is where the 

 rise in temperature is slowest, and this will occur when the por- 

 tion nearest to the thermoelement (see fig. 1) melts. A series of 

 melting point curves containing a typical one for each of the 

 observed feldspars, is reproduced here exactly as observed (see 

 page 114). The numbers represent the electromotive force of 

 the thermoelements at intervals of one minute, together with a 

 column of differences at the right of each record. The E.M.F. 

 will be seen to approach a minimum as melting progresses and 

 to increase again when it is complete. This minimum rise in 

 the temperature of course indicates the maximum absorption of 

 heat, For purposes of rough orientation 10 MY may be con- 

 sidered equivalent to one degree. 



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. 

 AVith AbaAiij it was barely discernible and with Ab 4 An x all 

 trace of the heat of fusion was lost.* Slow heating or rapid 



* 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.- 



