Properties of the Feldspars. 125 



considerable time afterward. Such a melting point would be 

 determinable only with the greatest difficulty, for all the func- 

 tions — mechanical, thermal or electrical — which usually become 

 suddenly discontinuous at the melting point would be equally 

 powerless to define a change of state in the face of such 

 extreme molecular inertia. 



In substances like these, which are still viscous at the tem- 

 perature of the electric arc, the sharpness of a minimum due 

 to heat absorption, for example, is not dependent upon the 

 magnitude of that absorption entirely, but also upon the 

 rapidity with which the change which involves it proceeds. 

 In albite and orthoclase, the velocity of this change is very 

 small. 



Specific Gravity. — The study of the specific gravities yielded 

 one interesting result which was not anticipated. The arti- 

 ficial feldspars, being chemically pure and homogeneous, gave 

 a perfectly definite specific gravity which could be determined 

 with great accuracy if the specimen was completely crystal- 

 lized. If vitreous inclusions were still present, the results were 

 of course variable and were all too low. It was anticipated 

 that the density of pure glasses, transparent and free from 

 bubbles, as they were in the more calcic members of the series, 

 might yield values varying more or less with the rate of cool- 

 ing, or after annealing, but this did not prove to be the case. 

 Our results did not vary more than two units in the third 

 decimal place in the same preparation even with the more 

 calcic feldspars which required to be very rapidly chilled in 

 order to cool the melt without crystallization. 



The determination of specific gravities is a trite subject, but 

 we have found the common methods liable to such grave 

 errors that we venture to give some useful details. The error 

 due to the evaporation of water about the stopper of the pyc- 

 nometer is very much less with finely ground stoppers than 

 with coarse grinding, and if the stopper is slightly vaselined 

 just before the final weighing, the error from this cause will 

 hardly affect the third decimal place with 25 cc pycnometers. 

 The simplest form of flask with a small capillary opening in 

 the stopper, is, in our judgment, far superior to one carrying a 

 thermometer. The temperature should be made sure by the 

 use of the thermostat. 



For removing the air from a powdered charge, we used the 

 device of G. E. Moore,* slightly modified, as indicated in the 

 accompanying sketch. The bulb A contains boiled water. 



*G. E. Moore, Jour, prakt. Chem., ii, p. 319, 1870. 



Am. Jour. Scl— Fourth Series, Vol. XJX, No. 110. — February, 1905. 

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