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IV. On a Method of Determining the Specific Gravity of 

 Small Quantities of Dense or Porous Bodies. By J. Joly, 

 M.A.B.E., Trinity College, Dublin*. 



IT may happen to be of importance to the mineralogist or 

 to the chemist to determine the specific gravity of a very 

 minute quantity of a solid. A method of effecting this, 

 whatever the density of the solid or whatever its state of 

 aggregation, is described in the following note. It, moreover, 

 calls for the use of no special appliance. The method now in 

 general use for the micro-determination of the specific 

 gravities of silicates, &c, of low density, is by balancing in a 

 liquid of a specific gravity adjustable to that of the specimen, 

 and subsequently determining the density of the solution 

 employed. 



This method fails altogether, 



(1) when the substance has a specific gravity over four ; 



(2) when the substance is of a porous nature. 



In the first case the method fails for want of a liquid of 

 sufficient density to equilibrate the solid. Be it observed 

 that about 90 per cent, of the unsilicated mineral species range 

 in density above 4*5. 



In the second case— the case of porous bodies — the im- 

 possibility of freeing the body entirely from contained air, 

 when immersed in liquids of the nature of those to which we 

 are restricted, renders the method fallacious. There is 

 another source of error. The particle in the first instance 

 soaks in a certain quantity of liquid at such a density, subse- 

 quently the density of the liquid surrounding it is brought to 

 another density. Is it assured that the density of the liquid 

 lodged in the interstices of the substance in the first instance 

 assumes the density subsequently conferred on the surround- 

 ing liquid ? It might need a considerable time for the desired 

 equalization of density to occur. 



Briefly, the theory of the present method is as follows : — 

 The mineral by itself will not float in any known solution, 

 suppose. If, however, we mix it with another substance of 

 much lower specific gravity, there is easily found such a pro- 

 portion for the constituents as will enable the mixed bodies 

 to be equilibrated by dilution of the specific gravity liquid. 

 We may, in short, adjust the specific gravity of the mixed 

 substances to be as close to that of either of them as we 

 please. 



We require to know — 



* Communicated by the Author. 



