126 Day and Allen — Isomorphism and Thermal 



When the apparatus is exhausted, the water is allowed to flow 

 back into the pycnometer containing the charge ; then by tap- 

 ping and warming with water at 40-50° 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 

 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 stopper 

 of the pycnometer. To obviate this, wipe out the neck with 

 filter paper before stoppering and burn it in the tared dish. 



Aspirajo 



Picnometer / J t 

 witk Charge 



Fig. 11. 



If the powder is very fine it is advisable to allow the filled 

 pycnometer to stand for some hours in the thermostat, in order 

 that suspended material may settle. With a 25 cc pycnometer 

 and 5 to 10 grams of material, this method usually yields con- 

 cordant results to the third decimal place, and the error from 

 all causes should never be greater than two units (± 1) in the 

 third place. A determination of this accuracy is of course sub- 

 ject to a correction for buoyancy, and all the numbers which 

 follow have been thus corrected. 



There is another error to which accurate specific gravity 

 determinations upon powdered minerals will be subject unless 

 suitable precaution is taken. The exposure to the air during 

 the period of grinding the samples gives opportunity for the 

 condensation of sufficient atmospheric 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 saturation of the atmos- 

 phere and the time of exposure. 



