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VII.— ON A METHOD OF DETERMINING THE SPECIFIC 

 GEAVITY OF SMALL QUANTITIES OF DENSE 

 OR POROUS BODIES. By J. JOLY, B.E., Assistant 

 to the Professor of Civil Engineering, Trinity College, 

 Dublin. 



[Read, January 20, 1886.] 



A method of determining the specific gravity of a small quantity of 

 a heavy mineral is often a desideratum in the course of inquiries into 

 the composition of rocks, sands, volcanic ash, &c. The mineralogist 

 is indeed frequently called upon to determine the nature of minerals 

 distributed .but sparsely throughout his specimen, or even when 

 abundant — from the intimateness of their intermixture with other 

 substances — only procurable in very small fragments, and, except 

 with the expenditure of much time and labour, in very small 

 quantities." The same case arises when it is not desirable to deface 

 an implanted specimen of rare beauty of form. Finally, the 

 chemist is often called upon to determine the physical properties 

 of minute quantities of matter, as in the case of the rare elements. 



Whether as a characteristic for discrimination, or as a physical 

 property to be placed on record, the quality of specific gravity is 

 of sufficient jimportance to justify me in calling your attention to 

 a method of determining it, specially applicable for dealing with 

 small quantities of very dense bodies, and also with small quanti- 

 ties of porous, fibrous, or very cleavable bodies. 



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 em- 

 ployed. 



This method fails altogether — 



(a) When the substance has a specific gravity— over four. 

 (&) 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. Indeed we cannot 



SCIEN. PROC, B.D.S., VOL. V. PT. II. E 



