92 
SEPARATION OF ROCK CONSTITUENTS. 
Apr., 1889. 
The solution I shall use is the double iodide of mercury 
and potassium which is made by dissolving 220 grammes of 
iodide of potassium in water, and adding to it 290 grammes 
of the binioaide of mercury. The scarlet iodide gradually 
dissolves, forming a yellowish liquid which is to be evaporated 
on the steam bath until it crystallises on cooling. The 
addition of a drop or two of water redissolves the crystals, 
and the solution may then by readily filtered through paper. 
It has a specific gravity of 3*196 ; that is to say, fluor spar 
floats on it. 
By substituting the iodide of barium for the potassium 
salt, the specific gravity may be increased to over 3*5. In this 
case the proportions are 100 of iodide of barium to 130 of the 
iodide of mercury. The two salts are heated and shaken 
with 20 of water until they are dissolved, and then concen¬ 
trated, if necessary, until topaz floats. In diluting the 
solution in the process of separation, an already diluted 
solution should be used to prevent a partial decomposition 
and deposit of the iodide of mercury, which is liable to occur 
when water is used. 
Another liquid used for the purpose is the borotungstate 
of cadmium, which can be obtained with the specific gravity 
of 3*6. Its preparation, however, is very troublesome, and in 
using it all carbonates must be previously removed as they 
decompose the substance. 
Biniodide of methylene has also been proposed as likely to 
be useful. It must be diluted with benzol, which may be 
evaporated off to obtain the substance ready for use again. 
The next point to be considered is the preparation of the 
specimen of the rock. 
This should be broken up in a mortar (a brass or bronze 
one of the tall shape is the best), and care must be taken to 
make as little actual fine powder as possible. By means of 
sieves the powder is separated and examined under the 
microscope, and the fraction must be chosen which is the 
largest grained ; one which is homogeneous. This will 
generally be such as will pass through a sieve of forty or fifty 
meshes to the inch. The very fine powder must be sifted out, 
say, through a 100 mesh sieve. 
Small beakers are taken, which should have a lip and hold 
about 1J to 2oz. 
Take about 30 c.c. of the solution and stir in the rock 
powder and a fragment of larger size of a mineral of known 
specific gravity to act as an indicator.” On settling, it will 
