Vol. 6$.~] CONSTANTS IN MINEKALS AND IGNEOUS EOCKS, 155 



These mineral-glasses were all colourless and transparent, with 

 the exception of pargasite which was pale-yellow, and aetinolite 

 which was olive-green ; they all, with the exception of apatite, 

 have well-defined zones in the different columns. 



The melting-points of the felspar-glasses were ascertained by 

 means of the meldometer. The results obtained seemed at first far 

 from satisfactory, since they appeared to have no relation to those 

 found by Prof. Joly, Mr. Cusack, or Prof. Dcelter, even allowing for 

 difference in purity of the specimens melted, and errors of experi- 

 ment. They were, however, found to lie on a regular curve, and 

 repeated trials still gave the same high temperatures. 



Some time after obtaining these results, my attention was 

 drawn to a paper by Messrs. Day & Allen, published in the 

 American Journal of Science (3). This paper gives an account of 

 the thermal properties of the felspars, the melting-points of which 

 were found to be considerably higher than any previously obtained. 

 A distinct comparison can be drawn between my results and 

 those recorded in Messrs. Day & Allen's paper, although the former 

 were determined independently and by a different method. 



The method employed by Day & Allen may be briefly described 

 as follows : — Chemically-pure soda-lime felspars were prepared 

 synthetically; these were powdered, and then placed in platinum- 

 crucibles of 100 cubic-centimetre capacity, which were slowly 

 heated to above 1400° C. in an electric furnace. 



The temperatures up to 1150° were taken with a gas-thermo- 

 meter ; above this point they were extrapolated by means of the 

 thermo-electric force developed on heating a thermo-element com- 

 posed of pure platinum and platinum with 10 per cent, of rhodium, 

 a curve was plotted showing the relation between time and tempera- 

 ture, and the position of an abrupt change in the form of the 

 curve, which denoted an absorption of latent heat, was taken to 

 represent the temperature at which fusion commenced. 



The melting-points were, by this method, found to lie on a straight 

 line, which showed a gradual fall of temperature towards the albite- 

 end of the series. 



The soda-lime felspars, then, since they comply with Dr. Fr. "W. 

 Kiister's rule that 



' the solidifying point of an isomorphous mixture lies on a straight line joining 

 the melting-points of the components, and can be calculated from the percentage- 

 composition of the mixture' (see also Zeitschr. fur Physikalische Chemie, 

 vol.viii,1891,p. 577). 



may be considered as isomorphous mixtures of pure albite with 

 pure anorthite, which correspond to Type I of Bakhuis Roozeboonrs 

 theoretical curves for those substances which mix in all proportions. 

 In the accompanying diagram (fig. 2, p. 156), wherein the ordi- 

 nate^ represent the temperatures at which fusion takes place, 

 and the abscissae the specific gravities of the crystalline substances, 

 the upper curve shows the relation between the two, found by 

 Messrs. Day & Allen with artificial felspars by the thermo-electric 

 method, and the lower dotted curve the relation found by means 



m 2 



