A good and serviceable imitation of silk is due to a simple and ingenious treatment of cotton. In 1845 John Mercer, a Lancashire calico printer, one day filtered a solution of caustic soda through a piece of cotton cloth. He noticed that the cloth, as it dried, was strangely altered; it had shrunk considerably both in length and breadth, had become stronger, with an increased attraction for dyes. This was the beginning of the mercerization which to-day produces cotton fabrics almost as strong and handsome as if silk. The cloth, preferably woven of long Sea Island staple, is immersed in a solution of caustic soda, and afterward washed in dilute sulphuric acid and in pure water. As it enters the caustic bath the cotton is pure cellulose, as it leaves the bath the fabric is hydrated cellulose, with new and valuable properties. The structural change in the fibre is decided. The original filament of cotton is a flattened tube, the sides of which are close together, leaving a central cavity which is enlarged at each edge of the surrounding tube. It is opaque and the surface is not smooth. The fibre has also a slight twist. The tube after treatment becomes rounded into cylindrical form; its cavity is lessened and the walls of its tube thicken; the surface becomes smooth and each fibre assumes a spiral form. Effects like these of mercerization are produced in paper as well as in cotton cloth, yielding vegetable parchment, a familiar covering for preserve jars and the like. Properties in Building Materials. Some sandstones, such as are common in Ohio and Indiana, soft when hewn in the quarry, soon harden on exposure to wind and weather; materials of this kind in early times afforded shelter more lasting than tents of boughs or hides. But the building art was to know a gift vastly more important when an artificial mud was blended of clay and water, with a steady improvement both in the strength and durability of the product. It was a golden day in the history of man when first a clayey paste was patted into a pot, a bowl, a kettle: then was laid the foundation of all that the potter, the brick maker, the tile molder have since accomplished. Another remarkable discovery, needing prolonged and faithful experiment, was reached when pottery was found to keep its form better when broken potsherds and bits of flint were mingled with its clay. A discovery of equal moment was that of mortar, probably approached in the daubing of mud or clay into chinks of stones, with the admixture first of one substance and then another until the right one was found, and the binder and the bound became of one and the same hardness. The Romans, a deliberate race, took two years in making a batch of mortar; that bond to-day protrudes from their walls as more resistant to the tooth of time than stone itself. Flame and Electricity as Modifiers.