ALTHOUGH Portland cement was first, produced in England in 1824, the product of that date can hardly be considered as belonging to even the same class of material which to-day bears the name. The present era of Portland cement came into being with the development of the rotary kiln, and its beginning may be considered as contemporaneous with the opening of the twentieth century. The product of today is materially superior to that of twenty years ago, largely because it is manufactured under more definite supervision. The mechanical rocesses involved in its making have been highly developed and the regulation of the raw material at all stages, from the quarry to the finished cement, has, with the single exception of the control of tlhe kiln temperatures, been brought to a high state of perfection. Notwithstanding these great improvements and developments, there has, however, been no substantial change in the inherent characteristics of the cement itself. The product, of to-day is more uniform in quality, is ground more finely, and develops somewhat greater strength, but its essential properties have, with the passing of the years, remained unchanged. No new cements have been announced until quite recently, as in the case of those based on a high alumina content.