56 ISOMORPHISM AND THERMAL PROPERTIES OP FELDSPARS. 



remaining are washed into a tared dish and finally weighed. It is 

 very important that not the smallest grain of material should get into 

 the ground joint between the neck and the stopper of the picnometer. 

 To obviate this, wipe out the neck with filter paper before stoppering 

 and burn the paper in the tared dish. If the powder is very fine, it is 

 advisable to allow the filled picnometer to stand for some hours in the 

 thermostat in order that suspended material may settle. With a 

 25 cc. picnometer and 5 to 10 grams of material, this method usually 

 yields concordant results to the third decimal place, and the error from 

 all causes should never be greater than 2 units ( 1) in the third place. 



Aspirator 



Picnometer 

 with charge 



Fig. 13. Apparatus for specific gravity determination. 



A determination of this accuracy is of course subject to a correction 

 for buoyancy, and all the numbers which follow have been thus 

 corrected. 



There is another error to which accurate specific gravity determina- 

 tions upon powdered minerals will be subject unless suitable precau- 

 tion is taken. The exposure to the air during the period of grinding 

 the samples gives opportunity for the condensation of sufficient atmos- 

 pheric moisture upon the grains to affect the weight in air. The 

 amount varies measurably with the size of the grains, as will be seen 

 from the accompanying data, and probably with the degree of satura- 

 tion of the atmosphere and the time of exposure. 



