Vol. 58. ] THE MINERAL ANALYSIS OF ROCKS. 167 
removed from the capillary of the stopper, or even sometimes before. 
If, however, there be a considerable quantity of the heavy powder, 
it will not at once leave the tube, but will form a plug stopping up 
the orifice; advantage may be taken of this, to save the inter- 
mediate step of collection in the receiver. Keeping the thumb on 
the opening of the canal in the stopper, the separator may be 
brought over a funnel containing a paper filter and the contents of 
the lower leg discharged at once into this. The heavy fluid which 
flows through is stored for subsequent use, and the first washings, 
made with carbon-tetrachloride, are also preserved, as they contain 
a considerable quantity of methylene-iodide. The powder having 
thus been brought onto a filter, either directly or from the receiver, 
the lower leg of the separator is washed by immersing its aperture 
below the surface of benzol contained in the receiver, and drawing 
this up by suction through the canal of the stopper. To complete 
the washing benzol may be driven from a wash-bottle through the 
canal of the stopper. ‘The powder which is thus extracted is added 
to that on the filter, the whole is well washed with benzol, and dried 
in a water-oven. It is then ready for weighing. 
The separator is next brought over another filter, the middle 
stopper very carefully removed, and the upper portion of the fluid 
with its floating grains allowed slowly to escape. After collecting 
the heavy fluid and the first washings with carbon-tetrachloride the 
separator is cleaned with benzol, supplied from a wash-bottle, into 
the filter. This cleansing also washes the powder in the filter, 
which is then dried and weighed. 
When all the minerals have been separated and their proportion 
determined by weighing, the analysis may be checked by comparing 
the specific gravity of the rock in bulk, with that calculated. from 
the specific gravity and proportion by weight of its components. 
according to the equation: 
5 (hn BSC 
D=100+ (7-+7-+7 ) 
where D is the density of the rock, A, B, C the proportions per 
centum of its component minerals, and d,, dj, d. their respective 
densities. 
From the mineral analysis of a rock its chemical composition can 
be calculated. Hence it follows that, in many cases, a mineral 
analysis may provide the data required in petrographical classi- 
fication. I was somewhat sceptical of its truth when I first 
reached this conclusion, but, in order to put it to a test which 
should be free from the temptations of personal bias, I requested 
my friend Mr. Teall, and afterwards Mr. Harker, to supply me 
with samples of rock which had already been chemically analysed, 
for investigation. They were to keep me in ignorance of the results 
of the chemical analyses until I had informed them of mine, cal- 
culated from the mineral separation, for comparison. The rock 
sent to me by Mr. Teall was a specimen, as he afterwards 
