120 ANNUAL REPORT 
MINERAL ANALYSIS OF CLAY BY HEATING WITH CALCIUM CARBONATE, 
Lunge and Schochor proceed on an entirely different basis and make 
use of the fact, more akin to the purpose in view, that silicates are unlocked 
by being heated in contact with CaO, as is clearly shown by the J. 
Lawrence Smith method of alkali determination. These investigators, 
though they did not work with clays proper, but with marls, succeeded 
in showing that the CaO attacks most readily the combined silica and 
the silica present in a fine state of division, thus giving a direct expression 
of the cement-1thaking value of the materials. By heating sufficiently, 
treating with hydrochloric acid and sodium carbonate the unlocked bases 
and the soluble silicic acid are removed, and the residue remaining is 
simply the matter which was not rendered soluble by the action of the 
CaO during the heating process. 
The work by Messrs. Lunge and Schochor in regard to this investiga- 
tion was very extensive, and before adopting their final method they 
made very thorough preliminary tests. The results obtained by them may 
be summed up as follows: 
1. The solubility of raw marls in acid is not a sufficient criterion for 
determining their hydraulic value because acids are not proper reagents 
for determining unlocked silica, since silicic acid itself is soluble in 
acid only in proportion to the amount of chemical water contained by it. 
The acid treatment does not discriminate between coarse and fine silica, 
between amorphous silica and crystalline silica, nor between feldspar and 
silica. 
2. However amorphous silica and quartz may differ otherwise, they 
show but little variation in behavior on burning with lime. The fineness 
of grinding is far more important. Although with the same degree of 
fineness and the same mode of mixing with equal amounts of CaO and 
heating under similar conditions, amorphous silica gives rise to a higher 
percentage of soluble silica than the quartz or a mixture of.the two kinds 
of acid, yet when ground sufficiently fine, even pure quartz is acted upon 
almost completely and much more so than amorphous silica not so fine, 
but still extremely fine from the ordinary standpoint. Since the activity 
of silicic acid depends upon its contents of chemical water, after ignition 
it does not differ from so much quartz. 
3. Chemically combined silica is completely unlocked. No matter 
how fine free silica is, it is always attacked less than silica in combina- 
tion and hence it is shown that the best source of silica for cement purposes 
is a silicate, as far as transportation of the silica to the lime is concerned. 
Lunge and Schochor have thus indicated a way by means of which 
it is possible to determine the per cent. of chemically active material in a 
marl, to separate the combined and very fine silica from the coarser quartz, 
thus permitting the statement that the higher the per cent. of silicate and 
very fine quartz is in a clay the higher is the value of the latter for hy- 
draulic purposes. In other words, the higher the per cent. of dissolved 
