These results show that the elongation per unit of length and weight somewhat decreases upwards towards the temperature of maximum strength, and thence decreases, so that whilst the elongation is nearly the same at all ordinary temperatures, it is more than doubled at red heat; 129 IV. ON THE COMPARATIVE VALUE OF VARIOUS KINDS OF STONE, AS EXHIBITED BY THEIR POWERS OF RESISTING COMPRESSION. (From the Memoirs of the Manchester Philosophical Society.) OUR knowledge of the properties of stone, viewed as a building material, is very imperfect, and our architects and stonemasons have yet much to learn concerning the difference between one kind of stone and another, both as regards their chemical constitution, their durability, and their powers of resisting compression. On this subject we have the experiments of Gauthey, Rondelet, and Rennie, which to some extent supply the deficiency and furnish data for the resistance to a crushing force of a considerable variety of stone. These are, however, to some extent inapplicable to the purposes for which such data are required; and not finding them in exact accordance with he results of some experiments recently made, I have ndeavoured to inquire into the causes of the discrepancy, nd to account for the difference. Stone is found in various forms and conditions, emedded in and stratified under the earth's surface. That ortion of it which is used for building purposes, is a ense coherent brittle substance, sometimes of a granuted, at others, of a laminated structure. These qualitie rying according to its chemical constitution and the which it has been deposited. Sometimes the lam' K and granular rocks alternate with each other; at others, a rock of a mixed form prevails, partaking of the characteristics of both structures. Independent of these properties is its power of resistance to compression, which depends chiefly upon its chemical combinations and the pressure to which it has been subjected whilst under the earth's surface from the weight of superincumbent materials. The granite also, and other igneous rocks, owe their hardness to their having crystallised more or less rapidly from a fused mass. In attempting to ascertain the ultimate powers of resistance of rocks which have been deposited by the action of water, it is necessary to observe the direction in which the pressure is applied, whether in the line of cleavage, or at right angles to it. In nearly all of the following experiments this precaution was attended to, and it will be seen that the strength is far greater when the force is exerted perpendicularly to the laminated surface, than when it is applied in the direction of the cleavage. In building with such stone, it is also important that it should be laid in the same position as that in which it is found in the quarry, as the action of rain and frost rapidly splits off the laminæ of the stone when it is placed otherwise. The strength of the igneous, or crystalline rocks, is the same in every direction, owing to the arrangement of the particles of which they are composed. It might have been advantageous to have ascertained, by analysis, the chemical composition of the substances experimented on; but as this varies in almost every locality, and that in accordance with the superincumbent and surrounding strata, this is of less consequence in practice than a knowledge of absolute facts in connection with the properties of the material. Deductions from direct experiment are of no small importance to the architect and builder, as he should not only be acquainted with the strengths and other properties of the material on which he works, but also with the changes of those qualities under the varied forms of stratified, metamorphic, and igneous rocks. On the durability of the specimens, I have made no further inquiry than in regard to their power of resistance to strain. Any addition would require a separate investigation into the chemical constituents of the different specimens, and into those changes to which stone of almost every description is subjected when exposed to the action of the atmosphere. In omitting this branch of the investigation I have not forgotten its importance, but have very properly left its development to abler hands. Before giving the results of the inquiry, I may observe that a portion of the experiments were undertaken at the request of Mr. E. W. Shaw, the surveyor of the borough of Bradford, in Yorkshire, in order to ascertain the best and strongest qualities of stone for paving the streets of that town. The following Tables give the result of the experiments on fifteen specimens of Yorkshire sandstone, and on some specimens from Wales and other places, as follow: Experiments to determine the force necessary to fracture, and subsequently to crush, 2-in. cubes of Sandstone from the Shipley quarries, Bradford. The pressure applied in the direction of the cleavage. No. of Experiment. 12 13 33,524 fractured. 16 |