622 Scientific Proceedings, Royal Dublin Society. 



the Petrological Laboratory in Trinity College. The rock under 

 examination is powdered till it all passes through the finest sieve 

 usually to be found in a chemical laboratory; sufficient is taken 

 from this for all purposes of chemical analysis ; the remainder is 

 placed in a tall beaker and washed with water. In this manner 

 the fine dust or flour is removed, and one obtains a larger quantity 

 of material, in relation to the quantity of rock pulverized, than if 

 one employed only the powder, which will not pass through the 

 finest sieve, with the additional advantage that it is of finer grain, 

 and thus affords purer separations. No loss of time is involved in 

 the use of finer powder ; indeed, from the more perfect separation 

 which it insures, there follows a slight gain in this direction. The 

 fine powder is then dried in the water-oven and introduced into the 

 separating-tube ; the powder, first separated, is well washed with 

 distilled water and dried, and its specific gravity taken by the aid 

 of the modified Sprengel's tube, described in the succeeding Paper 

 The powder, after removal from the Sprengel, is washed first with 

 ether, and then with absolute alcohol, dried, and, if necessary, 

 analysed ; a known volume of water is then added to the mixture 

 which remains in the separating-tube till another mineral com- 

 mences to fall, and the course of procedure just described is re- 

 peated. From the quantity of water added a good idea, useful 

 as a guide, is obtained as to the specific gravity of the fallen 

 powder, and, in some cases, this will be sufficiently near the 

 truth to render a more exact determination needless. 



From experience I can recommend this course of procedure both 

 for speed and accuracy. 



