394 FRAGMENTS OF SCIENCE. 



features in both : you see flakes clinging to the surfaces of 

 each, which have been partially torn away in cleaving. 

 Let any close observer compare these two effects, he will, 

 I am persuaded, be led to the conclusion that they are the 

 product of a common cause. 1 



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 1 this clay which seems so 

 uniform, and find that the fracture presents to my eyes in 

 numerable surfaces along which it has given way, and it 

 has yielded along those surfaces because in them the cohe 

 sion of the mass is less than elsewhere. I break this mar 

 ble, and even this wax, and observe the same result ; look 

 at the mud at the bottom of a dried pond ; look to some of 

 the ungravelled walks in Kensington Gardens on drying af 

 ter rain they are cracked and split, and, other circumstances 

 being equal, they crack and split where the cohesion is least. 

 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 polyhcdra bounded by surfaces of weak cohesion. 

 Imagine such a mass subjected to pressure it yields and 

 spreads out in the direction of least resistance ; a the little 



1 I have usually softened the wax by warming it, kneaded it with the 

 fingers, and pressed it between thick plates of glass previously wetted. 

 At the ordinary summer temperature the pressed wax is soft, and tears 

 rather than rlcaves ; on this account, I cool my compressed specimens in 

 a mixture of pounded ice and salt, and when thus cooled they split beau 

 tifully. 



2 It is scarcely necessary to say that, if the mass were squeezed equal 

 ly 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 



