SLATES. 395 



polyhedra become converted into laminae, separated 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 fis- 

 sures. 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 facili- 

 tate 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 biscuit 

 during this period without remarking the cleavage devel- 

 oped 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 anal- 

 ogy further. I invite you to compare the structure of this 

 slate, which was subjected to a high temperature during 

 the conflagration of Mr. Scott Russell's premises, with that 

 of a biscuit. Air or vapor within the slate has caused it 

 to swell, and the mechanical structure it reveals is precisely 

 that of a biscuit. During these inquiries 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 accidental cleavage, but this is 

 cleavage with intention. The volition of the pastry-cook 

 has entered into its formation. It has been his aim to pre- 

 serve a series of surfaces of structural weakness, along 

 which the dough divides into layers. Puff-paste in prepa- 



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 it quite defeated him in his effort to ob- 

 tain a granular powder. 



