188 
Proceedings of the Royal Society of Edinburgh. [Sess. 
silicates are decomposed by means of hot concentrated sulphuric acid, and 
the silica set free from them is brought into solution with hot dilute 
caustic soda solution. A circumstance which renders the method peculiarly 
suitable to Red Clays is the scarcity or absence in them of micaceous 
minerals, which are rather easily attacked by hot sulphuric acid. 
It cannot be pretended that a theoretically perfect separation is afforded 
by this means, since the extremely fine pumiceous and other mineral 
particles present in Red Clay are not likely to resist the attack of hot acid 
absolutely. Nevertheless, the procedure described below may be regarded 
as reasonably satisfactory, considering the variability of the material under 
examination. Preliminary experiments with No. 4 showed that treatment 
with hydrochloric instead of sulphuric acid does not completely remove 
the hydrous silicates ; e.g. in the case of No. 4 the residues amounted to 
about 40 per cent, instead of 30 per cent., and gave ignition losses of about 
3 per cent. Further, it was always found that the proportion of anhydrous 
residue obtained from a given sample by the sulphuric acid method is 
constant within 1 or 2 per cent, of the residue itself. Unduly prolonged 
treatment with the hot acid was found to be neither desirable nor necessary 
in the case of Red Clays. 
One gram of pulverulent material is mixed with 5 c.c. of water in a 
tall 150-c.c. beaker; 5 c.c. of concentrated sulphuric acid are added, and 
the mixture is evaporated down until fumes of S0 3 begin to come off freely 
(time, f-1 hour). The cooled magma is digested on the water-bath with 
100 c.c. of dilute (}- n.) hydrochloric acid during one hour with frequent 
stirring, and the beaker is allowed to stand in a slightly inclined position 
for at least six hours. After this the clear supernatant liquid can be decanted 
off to within a few cubic centimetres. The solid settlings are washed into 
a hot mixture of 35 c.c. of 10 per cent. NaOH solution and 25 c.c. of water 
contained in a larger beaker, and the whole is kept on the boil during five 
minutes. Settling then proceeds rapidly, and is complete within an hour. 
The clear alkaline liquid is poured off ; the residue is treated with 10 c.c. 
of concentrated hydrochloric acid, filtered upon a Gooch crucible, washed, 
dried, and weighed. 
The final treatment with hydrochloric acid is essential to a correct 
separation. An excessive proportion of sesquioxides, and especially of 
magnesia in the “ insoluble ” portion shown by some of the older analyses, 
may perhaps be due to neglect of this precaution. 
The united acid and alkaline extracts thus obtained contained the 
“ soluble ” or hydrous silicates. They were evaporated down and analysed, 
allowance being made for silica and alumina imported by the caustic soda. 
