SPNIND, CRO OUST, 283 
must result in a cheap process. But we find that this company uses as 
raw material a cement rock and limestone whose specific gravities are very 
close, and it is extremely doubtful whether their process would work 
with a limestone and soft clay, though it might be applied to a 
limestone-shale mixture provided their specific gravities were suf- 
ficiently alike, and even in this case serious doubts arise. With a 
cement-rock limestone mixture we have as a base a material already quite 
close to the Portland cement composition with all of the clay base incor- 
porated with most of the lime. Even if some segregation took place, the 
resulting mixture, especially when worked and blended, cannot be far 
from the required composition. 
Such a system could be used more generally if apparatus for intimate 
blending were invented, but so far no machine doing successful and cheap 
dry mixing has been devised, except for small quantities. There is some 
hope in this direction by the further application of pneumatic principles, 
using a differentiation of air blasts. 
In the grinding of the raw mixtures it is important to establish the 
proper rate of feeding and time of grinding by determining the insoluble 
residue on ignition as described in the chapter on chemical examination. 
Practically no insoluble residue should be left. In this manner we 
cannot only see that the mixture is intimate enough, but also that the 
grinding is not excessive, using more power than is necessary. 
In working coarse silicious clays or a clay base of soft clay and 
sandstone or sand good results cannot be obtained by grinding the com- 
plete mixture with the limestone in one and the same tube-mill, unless 
more tube-mills are installed and the feed decreased. It would seem 
the best policy in this case to grind the clays separately in a tube-mill for 
two reasons: first, to avoid the long grinding of the entire large mass, of 
which the limestone being the largest component is, as a rule, not so 
difficult to reduce; and, secondly, because the fine particles of limestone and 
the fine part of the clay would continually tend to envelop the harder 
fragments of quartz and thus prevent their complete reduction. The clay 
after its preliminary grinding would then be ground and blended with the 
limestone in the regular tube-mills. 
It has been frequently asserted by champions of the marl process 
that it is impossible to grind limestone and clay mixtures as fine as the 
marls occur in nature, and hence the product of the latter is bound to be 
intrinsically inferior. Also, it has been claimed that wet grinding will 
produce finer products than dry grinding, and that even the shapes of the 
quartz grains will be different when ground wet than when ground dry. 
In order to examine the relative fineness of the different raw mixtures, 
the writer undertook the mechanical analysis of a number of samples 
taken from various machines used in mills visited by him. ‘The analysis 
was made by the suspension method already described. 
