32 MR. W. FAIRBAIRN ON THE 



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 combi- 

 nations 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 crystallized more or less rapidly 

 from a fused mass. 



In attempting to ascertain the ultimate powers of resist- 

 ance 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 perpendicu- 

 larly 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 laminae of the stone when it is 

 placed otherwise. The strength of the igneous, or crystal- 

 line rocks, is the same in every direction, owing to the ar- 

 rangement of the particles of which they are composed. 



It might have been advantageous to have ascertained, by 

 analysis, the chemical composition of the substances experi- 

 mented 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 know- 

 ledge of absolute facts in connexion 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 proper- 

 ties 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. 



