32 Mr. J. Joly on the Specific Gravity of 



If the mixed solutions containing the pellet be left standing 

 for some hours before finally adjusting, it will be found on 

 examination with a lens that bubbles will no longer gather 

 on the paraffin. 



The last operation is finding the specific gravity of this 

 solution 7 which gives us 5 in the formula. This is most 

 accurately done in a Sprengel tube, holding about 2 cc. | 

 the bottle may also be used. 



The table opposite contains the results of some determina- 

 tions made by the method. 



Of experiments 4 and 5 it is interesting, perhaps, to note 

 that 4 was undertaken with the notion that the mineral 

 being dealt with was barite. Its weight, as a hand specimen, 

 was deceptive, it being penetrated by sphalerite. On getting 

 the result (2*78) it was concluded that an oversight had been 

 made somewhere in the measurements, and experiment 5 was 

 undertaken ; this giving 2' 7 7, the specimen was appealed 

 to. Tests then showed it to be calcite. The determinations 

 numbered 11 and 12 were effected on minute, greenish 

 crystals, hexagonal in shape, removed from the Wicklow 

 granite. The crystals alone sank rapidly in Thoulet's solution 

 (sp. gr. 2*77). The foregoing method was resorted to, and 

 the specific gravity of apatite being obtained, the substance 

 was tested in the usual way. The tests being confirmatory, 

 it was concluded that the mineral was apatite, a rare substance 

 in this granite. 



I have thought well to include in the table some of the 

 quantities obtained in working the formula, as bearing on the 

 scale on which the experiments have been made. It is 

 evident that the method can be applied on a smaller scale 

 still ; weighing to tenths of milligrams, 10 milligrams could 

 be dealt with. The method, it is seen, involves two weighings 

 and one determination of specific gravity by balancing in a 

 solution ; that is, if a piece of paraffin of known specific 

 gravity be at hand. 



Recently it has come to my notice that a method based on 

 a similar principle is described in Rosenbusclr's Mikro- 

 shopische Pliysiographie der Mineralien und Gesteine. In 

 detail, however, the methods differ essentially. Thus in 

 Rosenbusch's method the mineral is caused to adhere to a 

 piece of wax by pressure, and is subsequently taken off and 

 weighed after their joint specific gravity is determined. Here 

 the difficulty of freeing the substance from contained or 

 adhering air remains. A porous substance cannot be dealt 

 with. They differ, too, in other respects, so that I am induced 

 to think the foregoing may still, independently, be of use. 



