Page:Popular Science Monthly Volume 60.djvu/154

146 In the year 1756, the celebrated engineer, John Smeaton, was wrestling with the problem of constructing the Eddystone Lighthouse, While experimenting with different varieties of mortar, he discovered that certain limestones produced an hydraulic lime. He found that a mortar made of pure lime and pozzuolana or powdered bricks gave only unsatisfactory results, but when an impure lime from the 'Alberthaw' was used the hydraulic properties were more fully developed. Continuing his experiments, he at length announced that only limestones containing clay produced a lime of satisfactory hydraulic properties.

Speaking of this discovery, Smeaton says in his 'Narrative of the Eddystone Lighthouse':

It remains a curious question, which I must leave for learned naturalists and chemists, why an intermediate mixture of clay in the composition of limestone of any kind, either hard or soft, should render it capable of setting in water in a manner no pure lime I have yet seen, from any kind of stone whatever, is capable of doing.

It is easy to add clay in any proportion to pure lime, but it produces no such effect; it is easy to add brick dust, either finely or coarsely powdered, to such lime in any proportions also; but this seems unattended with any other effect than what arises from other bodies, becoming porous and spongy and therefore absorbent of water as already hinted and excepting what may reasonably be attributed to the irony particles that red brick dust may contain.

In short, I have as yet found no treatment of pure calcareous lime that renders it more fit to set in water than it is by nature, except what is to be derived from the admixture of trass, pozzuolana and some ferruginous substance of similar nature.

These investigations and the conclusions that he drew from them led Smeaton to use in the construction of the Eddystone Lighthouse a mortar or cement composed of hydraulic lime from the Alberthaw and Italian pozzuolana. A step or two farther in his investigations, which he did not take and which were not taken until the middle of the last century, would have led to the Portland Cement of the present time.

In 1796, a Mr. Parker, of London, patented a process for what he called 'Roman Cement." He used for this purpose certain nodules of limestone containing clay that were found along the coasts of the Isle of Sheppy and certain parts of Kent and Essex. These nodules were first calcined and then reduced to fine powder in mills. The result was a cement of a better quality than Smeaton's. In 1818, one Canvass White discovered and patented in the United States a process for making cement from a similar rock, found at Payetteville, in central New York. Large quantities were manufactured and used in the construction of locks on the Erie Canal, which was then being built. The State of New York purchased the patent and made it public property. This laid the foundation of a great industry, which is known generally