1. The use of a cementitious substance for binding together fragments of stone is older than history, and it is known that the ancient Romans prepared a mortar which would set under water. So far as our present knowledge of cement manufacture is concerned, however, the credit of demonstrating that a limestone containing clay possessed, when burned and ground, the property of hardening under water, is due to Mr. John Smeaton, who announced this as the result of his experiments made in 1756 in seeking a material with which to build the Eddystone Lighthouse. After this discovery by Smeaton nearly sixty years elapsed before M. Vicat gave the true explanation of this action, namely, that the lime during burning combined with the silica to form silicate of lime, the essential ingredient of hydraulic limes and cements.

In 1796, Parker, of London, obtained a patent for the manufacture of a cement from septaria nodules, and aptly named his product "Roman Cement." In 1824, Joseph Aspdin of Leeds, England, patented a process of manufacture of "Portland Cement".

2. The cements in general use in the United States to-day are of two kinds, Portland cements and natural cements, and in what follows our attention will be directed almost entirely to these two products.

Common limes were formerly used largely in engineering construction, but have of late been almost entirely superseded, for this purpose, by cements. Since the hardening of lime mortar depends on the absorption of carbonic acid from the atmosphere, these limes are sometimes called "air limes," while the hydraulic products which set under water are, for a similar reason, styled "water limes." Hydraulic limes, though playing an important role in foreign countries, are not manufactured or used to any extent in the United States. The European product known as "Roman" or "Vassy" cement, somewhat resembles our natural cement, but is usually inferior to the American article. Our chief interest in these products, which are used only abroad, is to know what relation they bear to the cements with which we are familiar. The following classifications are selected as being authoritative:

3. The conferences of Dresden (1886) and Munich (1884) on Uniform Methods of Testing for Materials of Construction, classified the hydraulic products as follows: —

(1) Hydraulic limes: made by roasting either argillaceous or siliceous limestones. They slake partially or wholly on the addition of water.

(2) Roman cements: made from argillaceous limestones having a large proportion of clay. They do not slake by the addition of water and hence must be mechanically ground to powder.

(3) Portland cements: obtained by burning to the point of insipient vitrification either hydraulic limestones or mixtures of argillaceous materials and limestones, and afterward grinding the product to fine powder.

(4) Hydraulic gangues: natural or artificial materials which do not harden alone, but which furnish hydraulic mortars when mixed with quicklime.

(5) Pozzolana cements produced by an intimate mixture of powdered hydrate of lime and finely pulverized hydraulic gangues.

(6) Mixed cements: the products of intimate mixtures of manufactured cement with certain materials proper for such a purpose. Mixed cements should always be designated as such and the materials entering into the composition should be stated, but it may be added parenthetically that these things are seldom done.

4. MM. Durand-Claye and Debray divide cements into six classes, namely, (1), Grappier cements — obtained by grinding the pieces of hydraulic lime which do not slake; (2), quick-setting (Vassy) cements — formed by burning very argillaceous limestones at a low temperature; (3), natural Portland cements, or those cements made from natural rock which correspond to artificial Portland in character; (4), mixed cements; (5), artificial Portlands; and (6), slag cements.

M. H. LeChatelier, an eminent French authority, divides hydraulic products into four classes, namely:1 — Portland cements, hydraulic limes, natural cements, and mixed cements. He subdivides the third class, natural cements, into quick-setting, slow-setting and grappier cements, and includes natural Portlands among the slow-setting natural cements. Slag cements, which are put in a separate class by MM. Durand-Claye and Debray, are included in "mixed cements" by M. LeChatelier.

5. Prof. I. 0. Baker gives a classification that is better adapted for use in this country than any of the above.2 He divides the products obtained by burning limestone, either pure or impure, into lime, hydraulic lime and hydraulic cements. He then sub-divides cement into Portland, Rosendale (preferably called natural) and Pozzolana.