(To the Editor.)

Sib.—Your correspondent, Mr John Dennison mining engineer, considers it desirable that I should inform him whether the teit of my cement recently made was " from tbe briquette being set ia water seven days, or dry." For hia information I may state that the briquette was immersed in water immediately after mixing, and kept there for seven days, and then tested. '1 he object of Mr Dennison's enauiry on this point does not appear very olearly, but I Imagine it to be to ascertain whether my cement is rightly denominate! Portland cement. Mr Dennison says: "Should such a result be obtained from one set of rocks, the name ia inapplicable. Portland cement being made from sundry clays, marl?, chalk, and Has limestone." Does Mr Dennison mean by this land the seotence is by no means free from ambiguity) that no cement 1b properly desoribod as " Port la d" if it happens to ne made from " one set of rocks 1" If he does lam wholly at a loss to understand such a statement coming Irom a mining engineer. It is a matter of common knowledge amongßt experts that Portland cement does not take its name from the fact of its ingredients being "one-set," or several "sets" of rocks, Portland cement Is so called because it resembles Portland stone: and the f aot that It is manufactured from clay and lime, or from marl alone or limestone alone, has not the remotest connection with its name. A comparative examination cf English-Port land cement and my own reveals an absolutely identical chemical analysis between the two, the same average weight per bushel and precise'y the same behaviour under water In both, and a very close likeness In colour. English Portland Cement contains 62 per cent. of dime, and 35 per cent, of silicates and iron. Gorman Portland Cement,! 63 per cent, lime, 32 per cent. BiUcates and iron; whilst my own can tains 63 per cent, lime, and 31 per cent, silicates and iron. Mr Dennison will, therefore, see that my cement is strictly and in every sense of the word a Portland Cement, and I could not with reason change its name as he suggests.— I am, &c, D. Atkinson.
