tions were performed
perfectly. It was only necessary to deliver the material at the top of
the chambers, and during its natural descent it was screened or dried as
the case might be.

All these inventions and devices, as well as those described in detail
above (except magnetic separators and mixing and briquetting machines),
are being used by him to-day in the manufacture of Portland cement, as
that industry presents many of the identical problems which presented
themselves in relation to the concentration of iron ore.

XVII. THE LONG CEMENT KILN

IN this remarkable invention, which has brought about a striking
innovation in a long-established business, we see another characteristic
instance of Edison's incisive reasoning and boldness of conception
carried into practical effect in face of universal opinions to the
contrary.

For the information of those unacquainted with the process of
manufacturing Portland cement, it may be stated that the material
consists preliminarily of an intimate mixture of cement rock and
limestone, ground to a very fine powder. This powder is technically
known in the trade as "chalk," and is fed into rotary kilns and
"burned"; that is to say, it is subjected to a high degree of heat
obtained by the combustion of pulverized coal, which is injected
into the interior of the kiln. This combustion effects a chemical
decomposition of the chalk, and causes it to assume a plastic
consistency and to collect together in the form of small spherical
balls, which are known as "clinker." Kilns are usually arranged with
a slight incline, at the upper end of which the chalk is fed in and
gradually works its way down to the interior flame of burning fuel at
the other end. When it arrives at the lower end, the material has been
"burned," and the clinker drops out into a receiving chamber below. The
operation is continuous, a constant supply of chalk passing in at one
end of the kiln and a continuous dribble of clinker-balls dropping
out at the other. After cooling, the clinker is ground into very fine
powder, which is the Portland cement of commerce.

It is self-evident that an ideal kiln would be one that produced the
maximum quantity of thoroughly clinkered material with a minimum amount
of fuel, labor, and investment. When Edison was preparing to go into
the cement business, he looked the ground over thoroughly, and, after
considerable investigation and experiment, came to the conclusion that
p