318 FRAGMENTS OF SCIENCE. 



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 n 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 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 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 granu- 

 lar 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 slid- 

 den ; 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 



We have now reached the termination of our task. 

 You have witnessed the phenomena of crystallisation, 

 and have had placed before you the facts which are found 

 associated with the cleavage of slate rocks. Such facts, 

 as expressed by Helmholtz, are so many telescopes 

 to our spiritual vision, by which we can see backward 



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

 and myself, see Philosophical Magazine for August, 1856. 



