. Discovery. Science. 104 DISCOVERY of coal or coke, with adequate transport facilities for send- ing away the finished product from the works. The usual method is to burn chalk containing the necessary calcium carbonate with the clay in cylindrical kilns, upwards of 200 ft. long, which slowly revolve. The kilns are inclined, enabling the mixture of chalk and clay, which is well pulverised and made into a liquid paste called " slurry," to travel slowly do\\Ti the kiln. At the lower end, a blast of coal-dust is blown in, which immediately catches fire and burns the on-coming slurrj^ t
. Discovery. Science. 104 DISCOVERY of coal or coke, with adequate transport facilities for send- ing away the finished product from the works. The usual method is to burn chalk containing the necessary calcium carbonate with the clay in cylindrical kilns, upwards of 200 ft. long, which slowly revolve. The kilns are inclined, enabling the mixture of chalk and clay, which is well pulverised and made into a liquid paste called " slurry," to travel slowly do\\Ti the kiln. At the lower end, a blast of coal-dust is blown in, which immediately catches fire and burns the on-coming slurrj^ to hard grey nodules of varying sizes up to a man's fist. The nodules, if suitably burned, are ground and reground value is believed to be tri-calcium siUcate ; therefore, if the percentage of calcium carbonate gets too low, too much bi-calcium silicate is formed ; whereas if it gets too higli, there is too much free or loosely combined lime present and the cement will be unsound. The general rule is that the higher the percentage of lime, the higher the strength of the cement ; but such factors as the fineness to which the raw materials are ground and the correct temperature of the kilns are most important. In a kiln one attempts to arrive at what is called incipient fusion. Modern methods of cement manufacture therefore ensure a cement of uniform quality from a particular. Fig. I.—hessi,e Output of chalk, with two diggers operated by five men iu all, totals 5,000 to 6,000 tons per week. to a powder so fine that, in order to qualify- for the British Standard specification, 86 per cent, of it must pass a sieve with 32,400 meshes to the square inch. In this state it is the finished Portland cement. At Barton the cement is ground so that approximately 97 per cent- passes this sieve. The whole process is, at the present day, carried out in the most scientific manner ; several chemists are employed in a large establishment. This ensures the product being not only good, b
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