Portland cement is, as is well known, composed of a mixture of chalk, or other carbonate of lime, and clay--such as is obtained on the banks of the Thames or the Medway--intimately mixed and then subjected to heat in a kiln, producing incipient fusion, and thereby forming a chemical combination of lime with silica and alumina, or practically of lime with dehydrated clay. In order to effect this, the usual method is to place the mechanically mixed chalk and clay (technically called slurry), in lumps varying in size, say, from 4 to 10 lb., in kilns with alternate layers of coke, and raise the mass to a glowing heat sufficient to effect the required combination, in the form of very hard clinker. These kilns differ in capacity, but perhaps a fair average size would be capable of producing about 30 tons of clinker, requiring for the operation, say, from 60 to 70 tons of dried slurry, with from 12 to 15 tons of coke or other fuel. The kiln, after being thus loaded, is lighted by means of wood and shavings at the base, and, as a matter of course, the lumps of slurry at the lower part of the kiln are burned first, but the moisture and sulphurous gases liberated by the heat are condensed by the cooler layers above, and remain until the heat from combustion, gradually ascending, raises the temperature to a sufficient degree to drive them further upward, until at length they escape at the top of the kiln. The time occupied in loading, burning, and drawing a kiln of 30 tons of clinker averages about seven days. It will be readily understood that the outside of the clinker so produced must have been subjected to a much greater amount of heat then was necessary, before the center of such clinker could have received sufficient to have produced the incipient fusion necessary to effect the chemical combination of its ingredients; and the result is not only a considerable waste of heat, but, as always occurs, the clinker is not uniformly burnt, a portion of the outer part has to be discarded as overburnt and useless, while the inner part is not sufficiently burnt, and has to be reburned afterward. Moreover, the clinker, which is of excessively hard character, has to be reduced by means of a crusher to particles sufficiently small to be admitted by the millstones, where it is ground into a fine powder, and becomes the Portland cement of commerce.