2 Day, etc. — Determination of Mineral 



ample, in the form of a cold glass), and the properties charac- 

 teristic of the substance in the form stable at this temperature 

 may thereby be completely veiled. 



In passing from the amorphous or glassy condition over to 

 the crystalline condition a compound usually undergoes a sud- 

 den change of specific volume. It is obvious that with rising 

 temperature a glass might increase in specific volume continu- 

 ously, and perhaps at the same time irregularly, in softening 

 gradually to its normal liquid state; yet the fact would indicate 

 nothing as to the amount or even the direction of its specific 

 volume change in passing from the amorphous (liquid) state to 

 the crystalline state. 



In the second place, it should be noted that a rock, which is 

 nearly always a mixture or solid solution, behaves very differ- 

 ently from a single chemical compound. The single compound 

 melts at a definite temperature, whether its liquid be viscous 

 or not, in accord with the above definition ; the rock liquefies, 

 continuously or in distinct steps according to the thermal char- 

 acteristics of its individual components, and through a range 

 of temperature which may be very wide. We cannot speak, 

 then, of the "volume change on melting" of a rock as though 

 it were a specific property, but only of the " volume changes 

 between the temperatures £,and £,." With rising temperature, 

 some of these changes of specific volume may be positive, some 

 negative; some will occur at one temperature, some at another; 

 and the total between temperatures t 1 and t Q may be an increase 

 or a decrease. 



Indirect Estimates. — The most of the estimates of the vol- 

 ume changes in rocks solidifying from an amorphous state are 

 drawn indirectly from the specific gravities at 20° of the glassy 

 and the crystalline rock respectively.* In general, the specific 

 volumes of silicate glasses at 20° have been found to be greater 

 than those of crystalline solids of the same composition. Hence 

 if the dilatation coefficients of the solid and the glass are nearly 

 equal, the rock may be assumed to have undergone a net ex- 

 pansion in liquefying. 



Estimates made from this standpoint are subject to several 

 large uncertainties. The unknown rate of volume dilatation 

 with increasing temperature, and the frequent changes in this 

 rate through inversions in the solid state, are large unknown 

 factors. Then, too, the glass sometimes cools with the inclo- 

 sure of fine gas bubbles which make its volume appear too 



*A. Harker, Natural History of Igneous Rocks, p. 158, 1909. F. Zirkel, 

 Lehrb. d. Petrogr. , i. p. 680-685, 1893. A. Delesse, Reeherches sur les Verres 

 provenant de la Fusion des Roches, Bull. Soc. Geol. France (2), iv, 1380, 

 1847. J. A. Douglas, On changes of physical constants which take place in 

 certain minerals and rocks on the passage from the crystalline to the glassy 

 state, Quart. Jour. Geol. Soc, lxiii, 145-161, 1907. 



