30 Mr. J. Joly on the Specific Gravity of 



W the weight of the mineral, 

 ay „ „ „ buoyant substance, 

 o- „ sp. gr. „ buoyant substance, 

 s „ „ „ mixed substances, 

 in order to determine S, the specific gravity required. 

 Then, as ^ weight 



"' o " "~ volume' 



W +<w « 

 orS = 



s 



Wo-5 



(W + ft))0- — ft)/ 



By this means, then, we can deal with bodies of any specific 

 gravity ; and, further, if for the buoyant substance we chose 

 one which, when brought to the liquid state, will creep into 

 and surround the substance, we may evidently be independent 

 of conditions of aggregation, and all trouble with contained air, 

 or bubbles adhering to the surface of a rough fragment, avoided. 



The method is practically carried out in the following 

 manner : — 



The specific gravity of a piece of translucent, homogeneous 

 paraffin, free from bubbles, is taken by any of the ordinary 

 methods — weighing in water with a sinker, or balancing in a 

 mixture of alcohol and water, and then determining the 

 density of the solution. The value found is what is called <r 

 above, the specific gravity of the buoyant substance. There 

 is no better paraffin for our purpose than that sold in the form 

 of candles. It fulfils all requirements. 



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

 3 or 4 mm. 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 homogeneous- 

 ness of the piece from which it is cut. There will be in 

 general, however, no need for so doing: 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 determined by 

 balancing in dilute alcohol : — 



(1) 0-9204 



(2) 0-9208 



— an inappreciable difference of specific gravity. The large 

 piece may be preserved for future determinations* 



