44 Scientific Proceedings, Royal Dublin Society. 



From this piece of paraffin a little disk-shaped piece — about 

 3 or 4 mms. in diameter, and 1*5 mm. thick — is cut with a sharp 

 knife, cleanly paired and smoothed on the edges by gently rubbing 

 between the fingers. The disk is larger or smaller according to 

 the quantity of mineral at our disposal, and if great accuracy 

 be desired we determine its specific gravity, thus avoiding any 

 assumption as to the homogeneousness of the piece from which it 

 is cut. There will be in general, however, no need of doing so : 

 thus compare the two following specific gravities obtained — (1) on 

 a piece of paraffin weighing over 11 grms. ; (2) on a little disk 

 removed from this, and weighing about '04 gram. (2) was deter- 

 mined' by balancing in dilute alcohol : — 



(1) 0-9204 



(2) 0-9208 



An inappreciable difference of specific gravity. 



The disk removed is next weighed in a delicate balance. If 

 as small as described above, the balance should read definitely to 

 0*2 mgr. Its weight is a> in the equation. It is in all cases 

 manipulated by use of a clean ivory forceps. If very minute it is 

 weighed on a tarred watch-glass, and so need not be manipulated at 

 all after preparation. Removed from the balance, the small frag- 

 ment (or fragments) of mineral is placed upon the surface of the 

 disk. The extremity of a slip of copper, about 5 mms. wide, is 

 now heated in a smokeless flame — it is better to use a little copper 

 ball, drilled and fitted on to a fine steel knitting-needle — and held 

 above the fragment of mineral, care being taken not to approach 

 it so closely as to endanger the paraffin being volatilized or of its 

 being melted so far as to risk loss by running over. Preferably 

 the disk of paraffin should rest on a piece of wet filter paper, or on 

 an anvil of clean copper ; this will keep the lower surface cool. 

 In point of fact, the mineral in general absorbing heat more freely 

 than the paraffin, melts the paraffin beneath it by conductivity, 

 and there is little risk of loss. The heating is continued till the 

 mineral is seen to be completely soaked with the paraffin — every 

 crack and cranny is then filled, the paraffin welling up and 

 swallowing the specimen and expelling all trace of air. 



