46 Prof. Tyiidall on the Cleavage of 



a high temperature during the conflagration of Mr. Scott Rus- 

 sell's premises. I invite you to compare this structure with 

 that of a biscuit ; air or vapour within the mass has caused it 

 to swell, and the mechanical structure it reveals is precisely that 

 of a biscuit. I have gone a little into the mysteries of baking 

 while conducting my inquiries on this subject, and have received 

 much instruction from a lady friend in the manufacture of pufF- 

 paste. Here is some such paste baked in this house under my 

 own superintendence. The cleavage of our hills is accidental 

 cleavage, but this is cleavage with intention. The volition of 

 the pastrycook has entered into the formation of the mass, and 

 it has been his aim to preserve a series of surfaces of structural 

 weakness, along which the dough divides into layers. Puff-paste 

 must not be handled too much, for then the continuity of the 

 surfaces is broken; it ought to be rolled on a cold slab, to 

 prevent the butter from melting, and diffusing itself through the 

 mass, thus rendering it more homogeneous and less liable to 

 split. This is the whole philosophy of puff-paste ; it is a grossly 

 exaggerated case of slaty cleavage. 



As time passed on cases multiplied, illustrating the influence 

 of pressure in producing lamination. Mr. Warren De la Rue in- 

 forms me that he once wished to obtain white-lead in a fine 

 granular state, and to accomplish this he first compressed the 

 mass : the mould was conical, and permitted the mass to spread 

 a little laterally under the pressure. The lamination was as 

 perfect as that of slate, and quite defeated him in his effort to 

 obtain a granular powder. Mr. Brodie, as you are aware, has 

 recently discovered a new kind of graphite : here is the substance 

 in powder, of exquisite fineness. This powder has the peculia- 

 rity of clinging together in little confederacies ; it cannot be 

 shaken asunder like lycopodium ; and w^hen the mass is squeezed, 

 these groups of particles flatten, and a perfect cleavage is pro- 

 duced. Mr. Brodie himself has been kind enough to furnish me 

 with specimens for this evening's lecture. I will cleave them 

 before you : you see they split up into plates which are perpen- 

 dicular to the line in which the pressure was exerted. This 

 testimony is all the more valuable, as the facts were obtained 

 without any reference whatever to the question of cleavage. 



I have here a mass of that singular substance Boghead 

 cannel*. This w^as once a mass of mud, more or less resembling 

 this one, which I have obtained from a bog m Lancashire. I feel 

 some hesitation in bringing this substance before you, for, as in 

 other cases, so in regard to Boghead cannel, science — not science, 

 let me not libel it, but the quibbling, litigious, money-loving 

 portion of human nature speaking through the mask of science 

 * For which I have to thank Mr. George Edraondson. 



