screening apparatus, and
believing that in the future engineers, builders, and contractors
would eventually require a higher degree of fineness, he determined, in
advance of manufacturing, to raise the standard ten points, so that at
least 85 per cent. of his product should pass through a 200-mesh screen.
This was a bold step to be taken by a new-comer, but his judgment,
backed by a full confidence in ability to live up to this standard, has
been fully justified in its continued maintenance, despite the early
incredulity of older manufacturers as to the possibility of attaining
such a high degree of fineness.

[Footnote 16: For a proper understanding and full
appreciation of the importance of fine grinding, it may be
explained that Portland cement (as manufactured in the
Lehigh Valley) is made from what is commonly spoken of as
"cement rock," with the addition of sufficient limestone to
give the necessary amount of lime. The rock is broken down
and then ground to a fineness of 80 to 90 per cent. through
a 200-mesh screen. This ground material passes through kilns
and comes out in "clinker." This is ground and that part of
this finely ground clinker that will pass a 200-mesh screen
is cement; the residue is still clinker. These coarse
particles, or clinkers, absorb water very slowly, are
practically inert, and have very feeble cementing
properties. The residue on a 200-mesh screen is useless.]

If Edison measured his happiness, as men often do, by merely commercial
or pecuniary rewards of success, it would seem almost redundant to state
that he has continued to manifest an intense interest in the cement
plant. Ordinarily, his interest as an inventor wanes in proportion to
the approach to mere commercialism--in other words, the keenness of his
pleasure is in overcoming difficulties rather than the mere piling up of
a bank account. He is entirely sensible of the advantages arising from
a good balance at the banker's, but that has not been the goal of his
ambition. Hence, although his cement enterprise reached the commercial
stage a long time ago, he has been firmly convinced of his own ability
to devise still further improvements and economical processes of greater
or less fundamental importance, and has, therefore, made a constant
study of the problem as a whole and in all its parts. By means of
frequent reports, aided by his remarkable memory, he ke