390 FRAGMENTS OF SCIENCE. 



ration 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 ho- 

 mogeneous and less liable to split. Puff-paste is, then, 

 simply an exaggerated case of slaty cleavage. 



The principle which I have 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 applica- 

 tion. When iron is taken from the puddling-furnace it is 

 more or less spongy, an aggregate in fact of small nod- 

 ules : it is at a welding heat, and at this temperature is 

 submitted to the process of rolling. Bright, smooth bars 

 are the result. But, notwithstanding the high heat, the 

 nodules do not perfectly 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 man- 

 ner 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 phenomena of which slaty cleavage 

 forms the most prominent example. 1 



[I would now lay more stress on the lateral yielding, 

 referred to in the note at the bottom of page 394, accompa- 

 nied as it is by tangential sliding, than I was prepared to 

 do when this lecture was given. This sliding is, I think, 

 the principal cause of the planes of weakness both in 

 pressed wax and slate-rock. J. T. 1871.] 



1 For some further observations on this subject by Mr. Sorby and 

 myself, see Philosophical Magazine for August, 1856. 



