Joly — On a Method of Determining Specific Gravity. 43 



We require to know — 



W the weight of the mineral, 

 to ,, ,, ,, buoyant substance, 

 g „ sp. gr. ,, buoyant substance, 

 s ,, „ „ mixed substances, 



in order to determine 8, the specific gravity required. 



Then, as 



weight 

 sp. gr. = — t-^ — , 

 volume 



£ = 



W 



W + 



io to 



or S 



s $ 



Was 



W+ to) a - toS 



By this means, then, we can evidently deal theoretically with 

 bodies of any specific gravity ; and, further, if for the buoyant 

 substance we chose one which, when brought to a liquid state, will 

 creep into and surround the substance, we may evidently be inde- 

 pendent of conditions of aggregation, and all trouble with con- 

 tained air, or bubbles adhering to the surface of a rough fragment, 

 avoided. 



How the method is practically carried out I now proceed to 

 describe. 



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 I called a- above, the 

 specific gravity of the buoyant substance. There is no better 

 paraffin for our purpose than that sold in the form of candles ; 

 nor do I see any reason to seek any other substance. It fulfils 

 all requirements, its penetrativeness when melted and its trans- 

 lucency when solid leave nothing to be desired. 



E2 



