96 Day and Allen — Isomorphism and Thermal 



Existing Methods, — Furthermore, the methods which have 

 been used in determining 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 constant. 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 deter- 

 minations 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 : 



Meldometer Measurement. Thermoelectric Measurement. 





Joly, 1891 



Cusack, 1896. 



Gas Furnace. 

 Doelter, 1901. 



Electric Furnace 

 Doelter, 1902. 



Microcline 



1175° 



1169° 



1155° 



1155° 



Albite 



1175 



1172 



1103 



1110 



Oligoclase 

 Labradorite 



1220 

 1230 



1235 



1110 

 1119 



1120 

 1125 



Anorthite 







1110 



1132 



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 differences between different feldspars. 



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

 fully 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 con- 

 cordant results with it to within about 5° C, but differences 

 several times greater than 5° appeared in the observations 

 made by one of us:f with the Joly apparatus, unless the grains 

 were prepared with the greatest care and all the observations 

 made by the same observer. The size and form of the grains, 

 the care used in locating them exactly in the middle of the 



* J. Joly, Proc. Roval Irish Acad., iii, 2, p. 38, 1891. R. Cusack, Proc. 

 Royal Irish Acad., iii,*4, p. 399, 1896. 



f C. Doelter, Tschermak, Min. u. Petr. Mitth., xx, p. 210, 1901; xxi, p. 

 23, 1902. t Day. 



