Flktcher — The Melting -Points of some of the Rarer Minerals. 157 



On the same basis the original figures obtained by Professor Joly would 



be for Von Kobell's scale — 



Orthoclase 1225 



Bronzite (Diallage) 1350 

 Quartz 1480 (.sf/tois 1460) 



Mierocliue 



Labradorito 



Albite 



1225 

 1280 

 1225 



Determination of Temperatures of Fusion on the Meldometer. 



The conditions pointed out by other observers whicli modify or determiue 

 the point of fusion of a mineral may be supposed to have been operative in 

 these experiments. 



The most important of such conditions are the rate of fusion of the 

 substance, and its purity. Joly,' Day, and others have shown that the time 

 element is very largely operative in modifying the fusion temperatures of 

 even pure substances. These observers' have shown that in addition to 

 the melting-temperature interval of impure substances tliere is a melting-time 

 interval during which the substance, if left for a sufficient length of time, will 

 fuse completely.'' The determination of the lower temperature limit of such an 

 interval may be impossible, and melting points therefore only approximate 

 " on account of the sluggishness " of the melting process. Fusion points, and 

 especially those determined on the meldometer, hence represent " the tempera- 

 tures at which the change is rapid enough to be observable within a 

 reasonable length of time." Mixtures and solid solutions (covering the case 

 of minerals) will not have melting-points but melting-intervals witli definite 

 temperature limits in which the substance will remain partially melted in 

 equilibrium. In the present work no attempt was made to assign the limits 

 of the melting-interval, as these must vary with composition, and hence 

 with locality. Tiiey probably lie closer to the upper limit of the interval 

 than to the lower. 



' Joly, Congrea Geologique Internationale, 1900. Comptes rendus, 1900, p. 689 ; also Soi. Proe. 

 Eoy. Dubl. Soc, vol. ix, 1900, p. 298. 



- Amer. Jour. Sci. (4), vol. xxxi, 1911, p. 185. 



See also Wegscheider (Chem. Zeit, 1905, vol. xxix, 1224), who points out that unless tempera- 

 ture rise be rapid the melting-point obtained may be that of the mineral and its decomposition products. 

 Cusack observed that with a sufficiently rapid rise of temperature certain minerals can be made to fuse 

 before decomposing. 



