334 FRAGMENTS OF SCIENCE 



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 compara- 

 tively 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 sub- 

 jected to pressure — ^it yields and spreads out in the direc- 

 tion of least resistance ; * the little polyhedra become con- 

 verted into laminae, separated from each other by surfaces 

 of weak cohesion, and the infallible result will be a ten- 

 dency to cleave at right angles to the line of pressure. 



Further, a mass of dried mud is full of cavities and 

 fissures. If you break dried pipe-clay you see them in 

 great numbers, and there are multitudes of them so small 

 that you cannot see them. A flattening of these cavities 

 must take place in squeezed mud, and this must to some 

 extent facilitate the cleavage of the mass in the direction 

 indicated. 



Although the time at my disposal has not permitted 

 me duly to develop these thoughts, yet for the last twelve 

 months the subject has presented itself to me almost daily 

 under one aspect or another. I have never eaten a bis- 

 cuit during this period without remarking the cleavage 



* 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 com- 

 pressed it. The mold was conical, and permitted the lead to spread out a little 

 laterally. The lamination was as perfect as that o^ alate, and it quite defeated 

 him in his effort to obtain a granular powder. 



