318 FKAGMENTS OF SCIENCE. 



surfaces of structural 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 melt- 

 ing, 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 aggre- 

 gate in fact of small nodules: it is at a welding heat, 

 and at this temperature is submitted to the process of 

 rolling. Bright smooth bars are the result. But not- 

 withstanding the high heat the nodules do not per- 

 fectly blend together. The process of rolling draws 

 them into fibres. Here is a mass acted upon by dilute 

 sulphuric acid, which exhibits in a striking manner 

 this fibrous structure. The experiment was made by 

 my friend Dr. Percy, without any reference to the 

 question of cleavage. 



Break a piece of ordinary iron and you have a 

 granular fracture; beat the iron, you elongate these 

 granules, and finally render the mass fibrous. Here are 

 pieces of rails along which the wheels of locomotives 

 have slidden; the granules have yielded and become 

 plates. They exfoliate or come off in leaves; all these 

 effects belong, I believe, to the great class of phenom- 

 ena of which slaty cleavage forms the most prominent 

 example.* 



"We have now reached the termination of our task. 



* For some further observations on this subject by Mr. Sorby 

 and myself, see Philosophical Magazine for August, 1856. 



