i8 



ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS. 



EXISTING METHODS. 



Furthermore, the methods which have been used hitherto in deter- 

 mining these mineral melting points seem to the authors to be open 

 to serious objection, both in principle and in application. They 

 depend, almost without exception, upon the personal judgment of the 

 observer, and not upon the actual measurement of any physical con- 

 stant. For this reason, perhaps, more than any other, the results 

 obtained by different observers upon the same mineral from the same 

 source do not agree within considerable limits, much larger than can 

 be properly ascribed to impurities in the specimens. Familiar 

 examples will best illustrate this point. Among the determinations 

 of the mineral melting points, two have received much more general 

 acceptance than others those of Joly* and of Doelter.f 



The melting temperatures which they obtained for some of the 

 typical feldspars are as follows : 



The determinations agree in recording higher melting points toward 

 the calcic end of the series, but the differences between corresponding 

 melting points by the two methods is greater than the observed differ- 

 ences between different feldspars. 



Joly's method was novel. He stretched a thin strip of carefully 

 prepared platinum foil between suitable clamps, placed a few grains 

 of the powdered mineral upon it, and mounted a small microscope 

 above, so as to be readily trained on any part of the strip. The foil 

 was then heated by an electric current which could be very gradually 

 increased, and the temperature measured from the linear expansion 

 of the strip at the moment when the observer at the microscope 

 noticed the first signs of melting. The author of this method was 

 able to obtain concordant results with it to within about 5 C, but 



* J- J ol Y. Proc. Royal Irish Acad., 3, 2, p. 38, 1891. R. Cusack, Proc. Royal 

 Irish Acad., 3, 4, p. 399, 1896. 



tC. Doelter, Tschermak Min. u. Petr. Mitth., 20, p. 210, 1901; 21, p. 23, 1902. 



