316 EEAGMENTS OF SCIENCE. 



But you will ask me how, according to my view, 

 does pressure produce this remarkable result? This 

 may be stated in a very few words. 



There is no such thing in nature as a body of 

 perfectly homogeneous structure. I break this clay 

 which seems so uniform, and find that the fracture 

 presents to my eyes innumerable surfaces along which 

 it has given way, and it has yielded along those surfaces 

 because in them the cohesion of the mass is less than 

 elsewhere. I break this marble, and even this wax, 

 and observe the same result ; look at the mud at the 

 bottom of a dried pond ; look at some of the ungravelled 

 walks in Kensington Gardens on drying after rain, 

 they are cracked and split, and other circumstances 

 being equal, they crack and split where the cohesion is 

 a minimum. Take then a mass of partially consolidated 

 mud. Such a mass is divided and subdivided by interior 

 surfaces along which the cohesion is comparatively 

 small. Penetrate the mass in idea, and you will see it 

 composed of numberless irregular polyhedra bounded 

 by surfaces of weak cohesion. Imagine such a mass 

 subjected to pressure, it yields and spreads out in the 

 direction of least resistance ; l the little polyhedra be- 

 come converted into laminae, separated from each other 

 by surfaces of weak cohesion, and the infallible result 



viously wetted. At the ordinary summer temperature the pressed 

 wax is soft, and tears rather than cleaves ; on this account I cool 

 my compressed specimens in a mixture of pounded ice and salt, 

 and when thus cooled they split cleanly. 



1 It is scarcely necessary to say that if the mass were squeezed 

 equally in all directions no laminated structure could be produced ; 

 it must have room to yield in a lateral direction. Mr. Warren De 

 la Rue informs me that he once wished to obtain white-lead in a 

 fine granular state, and to accomplish this he first compressed it. 

 The mould was conical, and permitted the lead to spread out a 

 little laterally. The lamination was as perfect as that of slate, and 

 Lt quite defeated him in his effort to obtain a granular powdf-r. 



