SPECIFIC GRAVITY 23 



easily arrived at and sufficiently accurate for practical purposes ; moreover, it is equally 

 useful in the case of fragments or even splinters of material so small that with other 

 methods the result would be unreliable. Unfortunately, however, the application of this 

 particular liquid is limited to the determination of stones whose specific gravity does not 

 exceed 3"3. It is true that other liquids are known having a greater specific gravity than 

 that of methylene iodide, but each has some disadvantage and fails to fulfil all the necessary 

 conditions mentioned above. 



The density of methylene iodide may be increased to 3"6 by dissolving in it iodine and 

 iodoform to the point of saturation. Stones with a specific gravity of 3"6 will then remain 

 suspended at any point in the liquid, while those that float can be made to remain suspended 

 by diluting the solution with benzene or pure methylene iodide. As before, the specific 

 gravity of the liquid can be determined by the help of a series of indicators, or by Westphal's 

 balance. The denser liquid obtained in the way j ust mentioned has the disadvantage of 

 being deeply coloured and almost opaque ; it is therefore difficult to observe the movements 

 of a stone immersed in it, and to determine at a glance its approximate specific gravity. 

 Nevertheless, in the absence of a better substitute it must be made use of, and is valuable 

 in certain cases. 



Recently, however, a liquid has been discovered even heavier than methylene iodide 

 saturated with iodine and iodoform, dense enough, in fact, for the determination of the 

 heaviest of precious stones. This is a double nitrate of silver and thallium, with the 

 chemical formula AgTl(N0g)2, which, although solid at the ordinary temperatures, melts 

 at 75° C. to a perfectly clear and transparent liquid with a mobility equal to that of water. 

 It has a specific gravity of about 4'8, and in it the heaviest of transparent precious stones, 

 namely, zircon, will float. It is miscible in all proportions with water, and hence liquids of 

 any required density can be obtained by simple dilution. When the liquid has been so 

 diluted that its specific gravity is identical with that of the stone under examination, its - 

 value can be found by means of Westphal's balance, or by the aid of a series of indicators, 

 the latter being the more convenient with this liquid. 



Such a determination presents rather more difficulty than one performed with methy- 

 lene iodide, inasmuch as the liquid must be kept at a certain temperature. For this 

 purpose the solid silver thallium nitrate is placed in a thin glass beaker of about the size 

 and shape of the vessel shown in Figs. 5 and 7, and heated on a water-bath or over a small 

 spirit or gas flame until it fuses. A little water is then added ; this not only lowers the 

 specific gravity but also the temperature at which fusion takes place, the melting-point 

 sinking to 60° or even to 50° C, a fact which adds considerably to the value of the liquid 

 for practical use. In diluting the liquid with water, great care must be taken to avoid 

 adding too much, since a small amount of water will make a considerable difference in the 

 density of the mixture. This mistake can be avoided by adding a little too much water at 

 first, and then driving off' the excess by evaporation, constantly stirring and watching the 

 behaviour of the stone, which will show when the density of the liquid becomes identical 

 with its own by remaining suspended at any point. Since the specific gravity of the liquid 

 varies not inconsiderably with the temperature, it is important that it should be determined 

 either by indicators or Ijy Westphal's balance at the temperature the liquid had when the 

 stone was observed to remain suspended in it. By means of this heavy liquid, then, the 

 specific gravity of the heaviest of precious stones may be determined with a sufficient degree 

 of accuracy, the only exceptions being those with metallic lustre, namely, iron-pyrites and 

 haematite, which are sometimes used for ornamental purposes. 



For the practical worker in precious stones, the determination of specific gravity is 



