SPECIFIC GRAVITY. 55 



In substances like these, which we found to be still viscous at the 

 temperature 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 artificial feldspars, being chemically 

 pure and homogeneous, gave a perfectly definite specific gravity which 

 could be determined with great accuracy if the specimen was com- 

 pletely crystallized. If vitreous inclusions were still present, the 

 results were of course variable and were all too low. It was antici- 

 pated that the specific gravity of pure glasses, even when 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 cooling, 

 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 ven- 

 ture to give some useful details. The error due to the evaporation of 

 water about the stopper of the picnometer 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. picnometers. 

 The simplest form of flask with a small capillary opening in the 

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

 mometer. 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 (fig. 13). The bulb A contains boiled water. When the appa- 

 ratus is exhausted, the water is allowed to flow back into the picnom- 

 eter containing the charge, then by tapping and warming with water 

 at 40 to 50 to produce boiling within, the air is effectively removed. 

 The material projected from the flask, if the boiling is violent, is then 

 washed back from the tube B with boiled water, and any small particles 



* G. E. Moore, Journ. prakt. Chem., 2, 319, 1870. 



M 



: 



