Crystals and Slate Rocks. 4i7 



— has so contrived to split hairs as to render the qualities of the 

 substance somewhat mythical. I shall therefore content myself 

 with showing you how it cleaves, and with expressing my conviction 

 that pressure had a great share in the production of this cleavage. 



The principle which I have enunciated is so simple as to be 

 almost trivial ; nevertheless, it embraces not only the cases I have 

 mentioned, but, if time permitted, I think I could show you 

 that it takes a much wider range. When iron is taken from the 

 puddling furnace it is a more or less spongy mass : it is at a 

 welding heat, and at this temperature is submitted to the pro- 

 cess 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 incorporate 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 experiment 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 elon- 

 gate 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 phsenomena of which slaty cleavage 

 forms the most prominent example f. 



Thus, ladies and gentlemen, we have reached the termination 

 of our task. 1 commenced by exhibiting to you some of the 

 phsenomena 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 



, ,a J n « " ^^^ *1^^ lio^ roared. 



Or the eagle soared." 



From evidence of the most independent and trustworthy 



* 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 pro- 

 pose. 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. 



