HINTS FOR OFFICERS IN THE DETECTION OF PRECIOUS STONES. 13 
all densities from 8 to 5 can be obtained, and as the salts are soluble 
in water the stones can be easily cleaned. This isnot a bad mixture, 
and very cheap, so might be used on anemergency. But I very strongly 
recommend that which follows, as it is “facile princeps” in ordinary 
work, 
“ Kren’s Bororunastate or CapMium.” 
This substance is a solid of the chemical composition (according to 
Professor Church). 
9 WO; BOs, 2 Cd 0, 2 HaO +16 Ag. 
If gradually heated (in a water bath, easily constructed anywhere) 
the crystals dissolve in their own water of crystallization, and form a 
liquid having, at about 170° Fahrenheit, a density of 3°55. 
In this liquid all the stones in Table III. above topaz will undoubtedly 
sink, and all below diamond float. Diamond and topaz will probably 
sink, but will be so near the density of the liquid as to afford the ex- 
perimenter a pretty good clue to their specific gravity. 
This experiment will very greatly clear the list for most. stones. 
But borotungstate of cadmium is capable of easy solution in water, 
and of being recondensed by heat. Dissolved in about +5 of its weight 
of water it yields a solution of density about 3:28. It can be tested by 
trying if it will just float a piece of hornblende. A stone floating in the 
3°55 solution, and sinking in the 3:28 must be sphene, peridot, epidote, 
diopside or axinite. The odds are greatly in favour of its being 
peridot. It is clear that if we carry about with us a set of crystals 
of known specific gravity we can narrow our limits to any extent. 
Dissolve the borotungstate in, say, its weight of water, throw your 
stone in, and condense by gentle heat till the solution is dense enough 
to bring the stone to the surface. Then try the density with your test 
crystals, and you will get a very close approximation to the specific 
gravity of your stone. If you cannot float it by concentrating the 
solution, try the above mentioned 3°55 solution. Ifit sinks in that, it 
is one of the stones above diamond in Table III., and you can carry 
the test further with chlorides of zinc and lead in a molten state. But 
it is not really necessary, as the hardness of the stone, and its form of 
crystallization—if found in crystals—will almost certainly identify it. 
Mr. Streeter would doubtless put up a set of test crystals of known 
specific gravity, or the officer could readily mark off a set for himself 
with the aid of a Nicholson’s hydrometer (vide Ganot’s Physics). This 
instrument would be of little use to take abroad, as its indications 
depend on so many corrections (temperature of water, etc.). The 
great advantage of the heavy liquid capable of variation of density 
is that it is independent of all corrections. 
I do not know whether the borotungstate of cadmium is obtainable 
in London—most likely it is. It is sold by Marquart, of Bonn, at a 
price of about £2 the kilogramme. It can of course be recrystallized, 
and used over and over again, and a very small quantity is needed for 
an experiment on such small stones as we are likely to find. 
Tf a stone is found in definite crystals, consult Table IV. Any 
