20 GENERAL CHARACTERS OF PRECIOUS STONES 



cup m' is immersed in the water in the vessel g until the reference mark at d is exactly in 

 the surface of the water ; the position of the reference mark at u on the vertical scale is then 

 read. To effect this, the eye of the observer is so placed that the reference mark o, which is 

 an acute triangle, exactly covers its reflection in the mirror, when the position of the upper 

 angle on the scale is read ; by this means error due to parallax is avoided. As 

 an example, let us suppose that o stands at division 45 on the vertical scale. The stone 

 to be determined is then placed in the upper cup m ; this will stretch the spiral spring, and 

 the stand h bearing the vessel of water must be lowered until o' again stands in the level of 

 the water. The new position of o in the mirror scale is now read ; let it be at division 75. 

 Then the weight of the stone in air corresponds to 75 — 45 = 30 scale divisions. The 

 stone is now placed in the lower cup m', which always remains immersed in water ; since the 

 weight is diminished the spiral will shorten. The stand h will now require to be raised 

 until d is again in the surface of the water ; suppose the reading on the scale is now 65. 

 The loss in weight of the stone in water is represented by 75 — 65 = 10 scale divisions 

 and the specific gravity is, therefore f^ = 30, corresponding to colourless, transparent 

 tourmaline. 



(5) Method lolth heavy liquids. — Within recent years this method for the determination 

 of specific gravity has become of considerable importance. It depends on the fact that a 

 body placed in a liquid will sink or float according as the density of the liquid is less or 

 greater than that of the body. A stone placed in a liquid heavier than itself will rise to 

 the surface ; if placed in a liquid lighter than itself it will sink to the bottom ; while 

 if the stone and liquid are of equal density the former will remain stationary at any point 

 within the latter. The movement upwards or downwards of a stone placed in liquid 

 is quicker or slower according as the difference in density between the stone and the liquid 

 is greater or less. 



The liquid for use in the application of this method must fulfil a variety of conditions. 

 First, it must be as heavy as possible, so that its use ma}' be extended not only to the lighter 

 gems, but also to as many as possible of the denser ones, which, if placed in a light liquid, 

 would merely sink to the bottom and render their discrimination impossible. Secondly, the 

 liquid should be clear, transparent, and colourless, so that there is no difficulty in observing 

 the movement of the immersed stone. Thirdly, it must not be thick or viscous, otherwise 

 the free movement of the stone would be impeded. Finally, it must mix readily and perfectly 

 in all proportions with a second lighter liquid, so that its density may be easily varied. 



All these conditions are fulfilled by methylene iodide, a compound of carbon, hydrogen, 

 and iodine having the chemical formula CHglj- It is one of the heaviest liquids known, 

 having at ordinary teniperatures a specific gravity of about 3'3. Owing to its high coeffi- 

 cient of expansion, its density varies, however, very considerably with the temperature ; at 

 10° C. it is 3-3375 ; at 15° C. it is 3-3265 ; and at 20° C. it is 3-3155. Methylene iodide is, 

 further, perfectly transparent, very mobile, and of a pale yellow colour. It is readily miscible 

 in all proportions with benzene, which is a light liquid of specific gravity 0'88. Thus by mixing- 

 methylene iodide and benzene together a series of liquids is obtained varying in density 

 between 0-88 and 3-3, the lower value being less than the density of water, and the hio-her 

 being three and a third times as great. 



To determine the specific gravity of a stone by this method, it is first placed in pure 

 methylene iodide contained in a tall cylindrical vessel, such as is shown in Figs. 5 and 7. 

 If the stone sinks, we know it is heavier than the liquid, that is, it has a specific gravity 

 greater than 3-3, but how much greater cannot be determined. If it remains suspended in 

 the liqufd, neither rising nor falling, even when moved about with a glass rod, we know that 



