SPECIFIC GRAVITY 25 



"better still, by the use of indicators. These indicators may conveniently be kept in the 

 bottles ; each should be chosen so that its specific gi-avity is near that of the liquid in 

 which it is to be kept. When the indicators show that the specific gravity of any one liquid 

 laas altered, this may usually be corrected by simply adding more benzene. 



A crystal or fragment of quartz (sp. gr. = 2-65) is a good indicator for liquid No. 4, and 

 should always remain suspended in the liquid. Liquid No. 3 may contain as indicators a 

 phenakite (sp. gr. = 2-95) and a rose-red tourmaline (sp. gr. = 3-02); the former will float 

 and the latter slowly sink if the liquid is of the correct specific gravity. In liquid No. 2 

 dioptase (sp. gr. = 3'29) should float and olivine (sp. gr. = 3'33) should slowly sink. Finally 

 in liquid No. 1 topaz (sp. gr. = 3"55) must float and hessonite (sp. gr. = 3'fi5) must sink. 



These four liquids enable us to classify for purposes of identification all precious stones 

 according to their specific gravity into five groups, namely : 



I. Stones with a specific gravity greater than 3-6. 

 II. „ „ „ „ between 3'3 and 3'6. 



III. ,, ,, ,, „ ,, o'O ,, o'o. 



IV. „ „ „ „ „ 2.65 „ 3-0. 

 V. „ „ „ „ of 2-65 „ less. 



The stones of group I. will sink in each of the four liquids, while those of other groups 

 will float in one or other of the liquids. 



As an example of the lielp such a series of liquids can give in the identification of a 

 precious stone, let us suppose that a doubt exists as to whether a colourless, transparent stone is 

 rock-crystal (sp. gr. = 2'65), phenakite (sp. gr. = 2'95), or colourless tourmaline (sp. gr. = 3'02). 

 If it remains suspended when placed in liquid No. 4 it must be quartz. If it sinks in liquid 

 No. 4 but floats in liquid No. 3 it will be phenakite ; if, on the other hand, it sinks in No. 3 

 it must be tourmaline. Supposing the unknown, colourless stone should be either diamond 

 (sp. gr. = 3'5) or zircon (sp. gr. = 4"65), then in the former case it will float in liquid No. 4, 

 and in the latter case it will sink. 



It is important that while observations are being made the liquids should be at the 

 ordinary temperature (15 — 20° C. = 59 — 68°|F.), and should not be subjected to any great 

 variations in this condition, since, as was noted above, the specific gravity of methylene 

 iodide is considerably altered by changes of temperature. 



In the third section of this book, which deals with the determination of precious stones, 

 full use will be made of these four liquids, and of the convenient classification of precious 

 stones into five groups, based on their differences in specific gravity. At this point it will 

 be useful to give a tabular list of the more important precious stones, arranged according to 

 their specific gravity from the heaviest to the lightest, and divided into the five groups as 

 determined by their behaviour in the four standard liquids. As already mentioned, the 

 specific gravity of any one precious stone shows small variations, which are indicated in the 

 table. 



