

Day and Sosman — Melting Points of Minerals. 343 



If, on the other hand, the substance yields the same constant 

 melting temperature with widely different rates of heating, 

 the melting temperature is a physical constant which is char- 

 acteristic of the substance and does not depend upon the con- 

 ditions of experiment. Diopside and anorthite are such 

 substances, and the published melting points of pure anorthite 

 and diopside to which criticism has been directed, were 

 obtained in this way. (See references 1, 4, 32, 44, 40.) 



If it be contended (as has recently been done by Dittler) 

 that the thermoelement does not give the true temperature of 

 the mineral at any time during the heating, or that the energy 

 change in the system is not contemporaneous with the dis- 

 appearance of crystal structure, experimental proof is readily 

 obtainable with the "quenching" furnace, which fixes the 

 matter beyond all doubt. 



Suppose the melting point of diopside, for example, to have 

 been observed, in the manner described above, at 1391°. Place 

 a portion (2 grams) of this crystalline charge in a quenching 

 furnace, heat it slowly to 1388° and hold the temperature con- 

 stant at that point for an hour, or until there can be no further 

 question that the thermoelement and all parts of the charge 

 have the same temperature ; then remove the bottom of the 

 furnace and drop the crucible with the diopside suddenly into 

 a basin of mercury, which has the effect of cooling it almost 

 instantly without giving the slightest opportunity for any fur- 

 ther change within the substance. If the crystalline structure 

 of the diopside, upon examination, is found to be unchanged, 

 1388° is below the melting temperature. Place the same mate- 

 rial in the furnace once more, heat to 1396°, hold this temper- 

 ature constant for some time and drop the charge quickly into 

 mercury as before. If the diopside now appears as a clear 

 glass, the change of state and all its attendant phenomena must 

 have occurred ^between 1388° and 1396°. 



By way of offering a visible record of this particular case, 

 this experiment has been performed in this laboratory with 

 both diopside and anorthite (chemically pure), and photographs 

 of the substances as they appeared after removal from the 

 furnace are reproduced in figs. 1 to 4. These photographs 

 with their accompanying data give absolute proof that the 

 melting point of the diopside was between 1388° and 1396°, 

 and the melting point of the anorthite between 1547° and 1561° 

 (new nitrogen-thermometer scale). 



The so-called melting interval of slow-melting compounds 

 (quartz, albite, orthoclase) should be carefully distinguished 

 from the " melting interval " of mixtures (lime-soda feldspars, 

 impure natural minerals). The former melt and become amor- 

 phous very slowly, but their melting interval is an interval of 

 time, whereas the melting interval of mixtures is an interval of 



