W. B. D. Edwards — On the Sejjaration of Minerals. 275 



The green sediment of tungstic hydrate is filtered off and washed, 

 the washings being added to the solution of barium borotungstate. 

 The liquid is evaporated down and allowed to stand. Yellow 

 crystals are formed, and with a little care these can and should be 

 obtained as single large crystals. The latter crystallize in two 

 forms, one as modified tetragonal prisms with well-developed 

 basal planes, and the other as flattened forms much resembling 

 hexagonal forms in shape. 



Nearly the whole of the barium borotungstate can be obtained, 

 the mother-liquid being evaporated down a little more after each 

 crop of crystals has been obtained. Towards the end of the process 

 transparent colourless platey crystals of barium borate may separate 

 out as well. The barium borotungstate crystals should be dissolved 

 up in water and recrystallized once again. They should then be 

 dissolved up in about 200 c.c. of distilled water and a solution of 

 CiSo^ added from a burette or a pipette, care being taken to add 

 it very slowly, drop by drop as long as a precipitate falls, and the 

 precipitate of BaSo4 is then filtered off and the filtrate is evaporated 

 down in a porcelain dish on a water bath till a piece of olivine 

 floats on the surface. This liquid will be found to have a specific 

 gravity of 3-46 at 60° F., and it takes some hours before some of 

 the salt crystallizes out and the specific gravity falls to 3-28. It 

 might thus perhaps be used for the separation of some minerals 

 of specific gravity greater than 346, though care would have to be 

 taken not to allow crystals to form on the lighter minerals and 

 thus sink them. 



It will be found that the cost of the chemicals used will come to 

 about three shillings, viz. : — 



450 grams (nearly lib.) sodium tungstate Is. 



675 ,, (nearly l^lb.) boric acid ^d. 



150 ,, (5^ ounces) barium chloride 4^<f. 



25 ,, (0-87 ounces) cadmium sulphate ... &d. 



Pure hydrochloric acid, filter papers, etc ^^d. 



The quantity of cadmium borotungstate obtained is about 160 

 grams, or 50 c.c. The borotungstate crystals separate out from the 

 solution saturated when hot and are nearly colourless. 



The crj'stals of barium borotungstate obtained earlier in the 

 process cannot be mistaken when once seen. They are light yellow, 

 transparent, and have a splendid lustre. The surface becomes yellow 

 and opaque on washing with water. 



The principal difficulty in the manufacture of this liquid is the 

 care that has to be taken in crystallizing the various salts out from 

 their solutions. There is no reason why any geologist with a little 

 knowledge of chemical manipulation should not make his own 

 heavy liquid, or why the manufacturer should charge such an 

 exorbitant price as at present, as the actual time spent in making 

 it is very small, though spread over several days. 



In conclusion, I must express my. obligations to Prof. Judd, F.R.S., 

 who has afforded me great facilities in carrying out these operations. 



