1919.] 
Departmental Reports. 
211 
methods. He used sulphuric acid of a determined strength to decompose 
the actual clay (H 4 Al 2 Si 2 0 9 ), claiming to leave all the other mineral 
matter unattacked. 
Other investigators, notably Schoene, perfected elutriation apparatus 
for separating the particles mechanically by a stream of water into a 
carefully graded series of fractions. This process was called “ mechanical 
analysis.”* * * § 
Experiments have since shown that elutriation alone cannot separate 
colloidal clay matter from the very fine particles of quartz and of other 
minerals accompanying an ordinary plastic clay. The method fails entirely 
with certain calcareous clays. Zschokke found an error of nearly 50 per 
cent, in the “ clay matter ” determined by mechanical analysis in such 
clays.f Nor has the Seger method of rational analysis been found satis¬ 
factory in general use, except for the purest forms of kaolin. Sulphuric 
acid has been found to decompose many minerals frequently associated 
with a plastic clay, strongly attacking even micas and feldspars if these 
are sufficiently fine.]: 
Method adopted in this Investigation. 
The method which appears most nearly to satisfy the above conditions 
was one developed and used by Zschokke§ in his general survey of the 
Swiss clayfields. It is based on the double process of sorting the materials 
into two fractions by elutriation, and of removing plastic clay (H 4 Al 2 Si 2 0 9 ) 
from associated finely divided rock minerals, &c., by the use of sulphuric 
acid. Details are given in his paper, but the following is a summary of 
the process. 
The clay is elutriated into two fractions—A, clay fraction, and B, sand 
fraction—which are then separately extracted with 10 per cent, hydro¬ 
chloric acid. 
Insoluble in HC1, 
treated with H 2 S0 4 
H&.) Soluble in HC1 
-n [(e.) Insoluble in HC1 
* {(/.) Soluble in HC1 
((c.) Insoluble in H 2 S0 4 : Quartz, feldspar 
mica, &c., determined by microscopic 
investigation. 
(■ d .) Soluble in H 2 S0 4 : Clay, reckoned by 
V difference. 
Estimated by difference. CaC0 3 , CaS0 4 , 
MgC0 3 , MgS0 4 , and FeO, determined 
by microscopic investigation. 
Coarser grains of (c). 
Estimated by difference—coarser grains of ( b ). 
Notes.— (1.) Extraction with HCl .—The material is dried and a known 
weight is gently rubbed with a pestle in a porcelain dish containing about 
100 c.c. of 10-per-cent. HCl until disintegrated. It is then treated for 
thirty minutes on a steam bath, filtered, and the residue washed, dried, 
and weighed. 
(2.) Extraction with H 2 SO ±.—About 25 c.c. of pure acid are run into 
20 c.c. of distilled water, and the acid thus diluted is used for the process. 
* Ashby, How to Analyse Clays, pp. 59 et seq. 
t B. Zschokke, Zur technischen Analyse der Thone, Bauw.alerialienlcunde, vol. 7, 
pp. 149-52, 165-70, 1902. 
$ B. Zschokke, loc. cit. 
§ Loc. cit. 
