STATE GEOLOGIST. 233 
cement of this composition in the rotary kiln. The writer had no difficulty 
in burning such a cement without resorting to high temperature test fur- 
naces. There is, however, some difficulty in keeping this up continually, 
since the high temperature would make the burning more expensive, 
the kilm linings subject to greater deterioration, and would increase the 
practical difficulties in operating the kiln. In addition, owing to the 
invariable fluctuations in the fineness of grinding, this high lime content 
would make longer storing necessary, which would likewise raise the 
cost. It might yet be mentioned that the above cement vitrifies thor- 
oughly at about 1400° C. 
Things become more serious, however, when we use a clay with 
a higher content of silica, say with the formula Al,O,6.66Si0,0.30Fe,O, 
2H,O, which is by no means unusual or arbitrary. In fact, there are 
many clays which show a much higher molecular proportion of silica. 
Since we know the lime content of the cement to be a function: of 4, 
the number of the silica molecules in the clay, it is bound to rise with 
the silica content of the clay. This clay, hence, would make a cement 
of the following composition: 
Per cents. 
TSMC ieck g aren ces SORE Sek oka ete ee et 69.83 
Sil Catrammateree mangas Otel Weavers HORE ho A Tents a bea 21.95 
JNINUUBTUODYE YES os Se ee ae Oe eR oa OED oe ECE ctr ems Eo 5.60 
JRE CRAG 5. SOs Os ae ae re en 2.63 
The sum of the lime and silica percentages is hence 91.78, which 
indicates that the fusible magma whose function it is to start the vitrifica- 
tion is very small in amount, not sufficient to bring about vitrification 
at temperatures within practical reach. This is, of course, shown still 
more strikingly by more silicious clays, so that no amount of fine grinding 
could produce a good cement at the temperatures practically obtainable. 
The formula 
«(3CaO-+SiO,) +-y(2CaO-LAl,O,). 
is therefore inherently wrong when applied indiscriminately in practical 
work, and must be corrected either by a change in the molecular pro- 
portions or by the use of a constant for the various degrees of acidity 
of the clays. 
_ Experiments on the Use of Different Formulae—In order to study 
the effect of the use of different formulas in the dry preparation of 
Portland cements, the writer made up synthetic mixtures of kaolin, 
fine ground quartz, calcium carbonate and ferric oxide in quantities of 
about 20 pounds of raw mixture, grinding the materials together in an 
iron ball-mill for five hours. ‘The iron oxide content was intended to be 
kept at about 3 per cent. These cements were then made up in five 
series of the following formulce: (3CaO)SiO,, (2CaO) Al,O,; (2.9CaO) 
