306 P'ff^f' Tyndall, on the Comparative View of [June 6, 



along which tlie 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 informs 

 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 late- 

 rally 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 peculiarity of clinging together in 

 little confederacies ; it cannot be shaken asunder like lycopodium ; 

 and when the mass is squeezed, these groups of particles flatten, 

 and a perfect cleavage is produced. Mr. Brodie himself has 

 been kind enough to furnish me with specimens for this evening's 

 discourse. I will cleave them before you : you see they split up into 

 plates which are perpendicular 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.f 

 This was once a mass of mud, more or less resembling this one, 

 which I have obtained from a bog in 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 — has so con- 

 trived 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 



* Cream cheese, at least such as I have tried, when torn asunder by the 

 fingers, shows a very perfect cleavage in planes perpendicular to the direction 

 in which the mass has been squeezed. In an ordinary loaf of household bread, 

 the portion near the under cinist may also be torn into laminae : this is perhaps 

 \)€St seen when the bread is fresh. 



t For which I have to thank Mr. George E^mondson. 



