64 ANNUAL REPORT 
cium carbonate only had 71.17 per cent. of silica rendered soluble in hydro- 
chloric acid and sodium carbonate solution, clay substance in the shape of 
washed kaolin, on being treated exactly like the quartz, was practically all 
taken into solution. Again, if we heat kaolin and calcium carbonate to- 
_ getherwe find that the reaction betweenthesetwo substances takes place at a 
much lower temperature than between very fine quartz dust or even silica 
prepared by precipitation from a sodium silicate solution. The expulsion 
of the combined water from the clay substance is soon followed by, or to 
some extent is even coincident with, the evolution of carbon dioxide from 
the calcium carbonate, so that at about goo° C. the clay substance, having 
lost the chemical water, has the latter replaced by calcium oxide, and in 
place of Al,O@,; 2510, 2H@ we have Al,O.2(Ca@siO;). 
Prof. Lunge and Dr. Schochor, two German investigators, showed 
ten years ago how much more available the combined silica of the clay 
substance is than even the amorphous silica obtained by precipitation. 
Practical experience in cement making demonstrates clearly that the 
more “clay” a material contains, the more readily does it vitrify a cement 
mixture, that is, the lower a temperature is required. 
It seems thus that combined silica in the shape of clay substance is 
the most efficient source of silica for the production of the lime silicates 
required in cement-making, viewing the subject from the chemical stand- 
- point. 
If pure kaolin were used for a cement mixture, there would be intro- 
duced, of course, a certain amount of alumina; for every part of silica 
we would have 0.86 part of alumina, the ratio of silica to alumina being 
46.3 : 39.81.16 to 1.00. Without going further at present, let it suffice 
to say that this brings in too much alumina and hence the problem of in- 
troducing readily available silica for the union with the lime cannot be 
solved by the simple expedient of using a pure clay, high in clay sub- 
stance. Though as much silica as possible should be brought in as com- 
bined silica, the alumina content must be kept within the bounds prescribed 
by experience, and the balance of the silica must be introduced as free 
silica or quartz. It should be as fine as possible, so that nature’s grinding 
process is utilized, saving the expense of artificial grinding. In some 
text-books, it is prescribed that a clay for cement-making should contain 
no free silica and that its alumina content should be about one-fourth of 
the silica content. This evidently is an absurdity, as for every part of 
silica the clay substance contains 0.86 part of alumina. 
Feldspar.—The second constitutent almost invariably present in clays 
is feldspar, in the shape of more or less decomposed fragments, still re- 
maining from the incomplete decomposition of the original rock or brought 
in from other rocks by way of water transportation. The particles, ac- 
cording to the degree of grinding which they have undergone, are 
coarser or finer, like the particles of sand. Asa rule they are finer than the 
