PHYSICAL PROPKUTIES 203 



When all precautions are taken, this method will yield results 

 accurate to 1, in the fourth decimal place. 



When the substance is soluble in \vater or affected by water, 

 xiiiie other liquid than water is used the specific gravity of which 

 has been determined, as benzene or carbon tetrachloride. The 

 experiment is carried out as with water, but the result must be 

 multiplied by the specific gravity of the fluid used in order to re- 

 duce it to terms of water as t he unit. 



III. The suspension method. The Westphal balance and the 

 u-e of heavy liquids. The advantage of this method is that the 

 gravity of small crystals or fragments may be determined with 

 accuracy to the third decimal place as in the pycnometer method. 

 The method consists in placing the fragment in a liquid, the spe- 

 cific gravity of which is higher than that of the fragment, which 

 will then float on the surface of the heavy liquid. 



The fluid is now diluted carefully with a lighter one, stirring after 

 each addition; a point will be reached when the fragment of 

 mineral will neither float nor sink ; the specific gravity of the liquid 

 and fragment are under these conditions identical and the fragment 

 remains suspended. The specific gravity of the fluid is now deter- 

 mined with the Westphal balance or the pycnometer when accu- 

 racy is required. 



The heavy solutions in general use are : 



Thoulet's solution. A solution of potassium iodide, KI, 1 

 part, and mercuric iodide, HgI 2 , 1.24 parts, dissolved in an excess 

 of distilled water. The solution is then evaporated on the water 

 bath until a fragment of fluorite floats on it ; on cooling, the gravity 

 rises to the maximum 3.196. It is then filtered, and well corked, as 

 on exposure to the air it absorbs water and its specific gravity 

 will fall to 3.1, where it remains constant. It is a convenient solu- 

 tion, as it may be diluted with water to any extent, and if left stand- 

 ing exposed, the water will evaporate until the specific gravity 

 reaches 3.1 ; by evaporation on the bath it may again be brought 

 back to 3.196. If darkened by the separation of iodine, this may 

 be corrected by adding a little mercury during the evaporation. 

 Methylene iodide, CH 2 I 2 , G = 3.315 at 20 C.,-is a little heavier 

 than Thoulet's solution, but has the disadvantage of not being 

 miscible with water, and benzole must be used to dilute it. It does 

 not attack metals as does the mercury solution. 



The heaviest solution of all is that suggested by Penfield, silver 

 thallium nitrate, which is liquid at 75 and has a specific gravity 

 of 4.5 and may be diluted with water. 



