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VIII. 



ON A METHOD OF DETERMINING THE SPECIFIC GRAVITY 

 OF SUBSTANCES IN THE FORM OF POWDER. By 

 W. F. SMEETH (Petrological Laboratory, Trinity College, 

 Dublin). 



[Communicated by Professor Sollas.] 



[Eead February 14, 1888.] 



The great difficulty in determining the specific gravity of a 

 powdered substance is owing to the tendency which the finer 

 particles have to float on the surface of the water. This difficulty 

 has been avoided in various ways by using other liquids, such as 

 paraffin, to sink the powder in. All these methods, so far as I 

 know, involve the determination of the specific gravity of the liquid 

 so used. In the method which I am about to describe, the specific 

 gravity of the vehicle used (which is ordinary vaseline) need not 

 be determined. 



All that is necessary for the operation is a little pan, which may 

 be most conveniently made with a light watch-glass, into which a 

 handle of fine platinum wire may easily be fused. The process of 

 making an experiment is as follows : — 



Place the pan on the top of an oven, and put into it, with the 

 point of a knife, a little vaseline. This quickly melts to a clear 

 liquid, and any air-bubbles which remain floating on the surface 

 may be removed by touching them with a piece of filter paper. 

 Now transfer the pan to a dessicator, to keep it free from dust, &c, 

 and allow it to cool. When cold, take it out, and suspend in a 

 beaker of water, by a fine wire, from the arm of a balance. The 

 weight in water, w^ of the pan and the vaseline is thus determined. 

 Now take it out of the beaker, holding it up sideways, so as to let 

 the water run off. If any drops remain, they may be removed 

 with a little filter paper, care being taken not to disturb the vaseline. 

 Now place the pan on the oven again, and when the vaseline has 

 melted, sprinkle over its^surface some of the powder whose specific 



