Beams of cement‐stone and cement‐mortar‐stone with the ratio of cement to sand of 1 : 3 were allowed to bend under their own weight, the cross section of the beams being 2.27 cm square and the distance between the supports 76.1 cm. The deflections were determined at the central point between the supports by means of an Ames gauge reading to 1&mgr;; the experiments were extended over a period of seven months. The curve of the deflectionvs.time generally starts as a parabola having a vertical tangent but beyond a certain point the curve becomes linear. The slope of the curve is a measure of themobilityof the material. The parabolic curve is due largely to the hardening of chemical origin, resulting from the absorption of water, which causes a gradual decrease in the mobility, but to a smaller extent to an elastic after‐effect. The mobility of the cement is much greater than that of the mortar and it decreases with the degree of curing. Theyield valueof the mortar is smaller than 45×105dynes/cm2(65 lbs./in.2) and the yield value of the cement was not perceptible with the apparatus used. For all practical purposes, cement‐mortar‐stone may be regarded as a solid of very low rigidity and cement‐stone as a very viscous liquid. The curvature of the stress‐strain curve, usually observed, is due to elastic after‐effects arising from the fact that the experiments require time. The modulus of elasticity is smaller in the case of cement (230–260,000 kg/cm2) than in the case of mortar (270–300,000 kg/cm2); it increases with the number of curing days and approaches a constant value.