1856.] the Cleavage of Crystals and Slate Mocks. 307 



furnace it is a more or less spongy mass : it is at a welding heat, 

 and at this temperature is submitted to the process of rolling : 

 bright smooth bars such as this are the result of this rolling. But 

 I have said that the mass is more or less spongy or nodular, and, 

 notwithstanding the high heat, these nodules do not perfectly in- 

 corporate with their neighbours : what then ? You would say that 

 the process of rolling must draw the nodules into fibres — it does 

 so ; and here is a mass acted upon by dilute sulphuric acid, which 

 exhibits in a striking manner this fibrous structure. The experi- 

 ment was made by my friend Dr. Percy, without any reference to 

 the question of cleavage. 



Here are other cases of fibrous iron. This fibrous structure is 

 the result of mechanical treatment. Break a mass of ordinary iron 

 and you have a granular fracture ; beat the mass, you elongate 

 these granules, and finally render the mass fibrous. Here are 

 pieces of rails along which the wheels of locomotives have slided ;* 

 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 phaenomena of which slaty cleavage forms the most promi- 

 nent example.f 



Thus, ladies and gentlemen, we have reached the termination 

 of our task. I commenced by exhibiting to you some of the 

 phaenomena of crystallization. I have placed before you the facts 

 which are found to be associated with the cleavage of slate rocks. 

 These facts, as finely expressed by Helmholtz, are so many tele- 

 scopes to our spiritual vision, by which we can see backward through 

 the night of antiquity, and discern the forces which have been in 

 operation upon the earth's surface 



" Ere the lion roared, 

 Or the eagle soared." 



From evidence of the most independent and trustworthy cha- 

 racter, we come to the conclusion that these slaty masses have been 

 subjected to enormous pressure, and by the sure method of experi- 

 ment we have shown — and this is the only really new point which 

 has been brought before you — how the pressure is sufficient to 

 produce the cleavage. Expanding our field of view, we find the 

 self-same law, whose footsteps we trace amid the crags of Wales 

 and Cumberland, stretching its ubiquitous fingers into the domain 

 of the pastrycook and ironfounder ; nay, a wheel cannot roll over 



* For these specimens and other valuable assistance I am indebted to Mr. 

 Williams. 



t An eminent authority informs me that he believes these surfaces of weak 

 cohesion to be due to the interposition of films of graphite, and not to any 

 tendency of the iron itself to become fibrous ; this of course does not in any 

 way militate against the theory which I have ventured to propose. All that 

 the theory requires is surfaces of weak cohesion, however produced, and a 

 change of shape of such surfaces consequent on pressure or rolling. 



