242 FRA GMENTH F SVfENCfS. 



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 

 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 biscuit 

 during this period without remarking 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 invite you to compare the struc- 

 ture of the slate, which was subjected to a high tempera- 

 ture 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 pastrycook lias entered into its formation. It has 

 been his aim to preserve a series of surfaces of struc- 

 tural weakness, along which the dough divides into layers. 

 Puff-paste in preparation must not be handled too much; 

 it ought, moreover, to be rolled on a cold slab, to 

 prevent the butter from melting, and diffusing itself, thus 

 rendering the paste more homogeneous and less liable to 

 split. Puff-paste is, then, simply an exaggerated case of 

 slaty cleavage. 



The principle here enunciated is so simple as to be 

 almost trivial; nevertheless, it embraces not only the cases 

 mentioned, but, if time permitted, it might be shown you 

 that the principle has a much wider range of application. 

 When iron is taken from the puddling furnace it is more 

 or less spongy, an aggregate in fact of small nodules: it is 

 at a welding heat, and at this temperature is submitted to 

 the process of roiling. Bright smooth bars are the result. 

 But notwithstanding the high heat the nodules do not 



