SLATES. 317 



little polyhedra become converted into laminae, sepa- 

 rated from each other by surfaces of weak cohesion, 

 and the infallible result will be a tendency 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 develope 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 biscuit during this period without re- 

 marking the cleavage developed by the rolling-pin. 

 You have only to break a biscuit across, and to look at 

 the fracture, to see the laminated structure. We have 

 here the means of pushing the analogy further. I in- 

 vite you to compare the structure of this slate, which 

 was subjected to a high temperature during the confla- 

 gration of Mr. Scott Russell's premises, with that of a 

 biscuit. Air or vapour within the slate has caused it 

 to swell, and the mechanical structure it reveals is pre- 

 cisely that of a biscuit. During these enquiries I have 

 received much instruction in the manufacture of puff- 

 paste. Here is some such paste baked under my own 

 superintendence. The cleavage of our hills is acci- 

 dental cleavage, but this is cleavage with intention. 

 The volition of the pastrycook has entered into its 

 formation. It has been his aim to preserve a series of 



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

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

 it quite defeated him in his effort to obtain a granular powder. 



