Page:Popular Science Monthly Volume 60.djvu/156

148 sand and water to the consistency of mortar and used. Like natural cement, Portland cements are hydraulic, and they make the strongest mortars known.

A modern street consists of a foundation of broken stone that is formed into a concrete or solid mass of masonry by admixture with mortar. The character and quality of this mortar are a matter of the greatest importance. All of the four kinds of mortar mentioned above have been used for this purpose.

An experiment was made in London by laying in Holborn, opposite Gray's Inn, nine inches of lime mortar concrete with a floating on top of inches of lime mortar. Upon this foundation was laid a surface of Val de Travers asphalte. When the concrete was ready to receive the

asphalte, a fire broke out in Holborn; the place was flooded with water, the engines drove over the concrete and the population of Gray's Inn trampled it down. It was subsequently made good and the asphalte spread. Tor five or six years the road was kept up at considerable expense and then relaid. On removing the asphalte, it was found that the lime concrete had never set, that the mortar floating had never adhered to the concrete, but was mostly in powder, produced either by the action of the rammers or by the traffic afterwards.

Roman cement was tried in Paris and condemned for street foundations. There remains for use for this purpose natural cement