20 Mr. H. C. Sorby on Slaty Cleavage, 



not foimd a great divergence from this result, although with 

 regard to the quality and quantity of the foreign substances in- 

 troduced, the divergences were considerable. 



The expansion of pure bismuth by solidification I have not 

 determined. This much, however, appears to follow from the 

 experiments here communicated, that it is far less than inaccu- 

 rate experiments have hitherto induced us to estimate it. 



[The interesting experiments of Dr. Schneider leave us in doubt 

 as to whether bismuth expands at all during solidification. The 

 following fact may perhaps throw some light upon this point. 

 An iron bottle was completely filled with the liquid metal and 

 closed with a screw tap ; on cooling, the bismuth burst the bottle, 

 and showed itself in a row of shining spheres along the rent. 

 It might be urged that this was caused by the contraction of 

 the iron, but the fact that bismuth possesses a higher coefficient 

 of expansion than iron makes against this supposition. — Eds.] 



III. On Slatij Cleavage, as exhibited in the Devonian Limestones 

 of Devonshire. By Henry Clifton Sorby, F.G.S.*^ 



IS slaty cleavage the result of crystalline or mechanical action? 

 This is a question I shall now attempt to answer by a de- 

 scription of the phsenomena to be observed in the Devonian 

 limestones of the neighbourhoods of Ilfracombe, Plymouth, and 

 Torquay. I do not think any rocks could be found more suit- 

 able for deciding it, because they present us with cleavage in 

 every state of development, from altogether absent to in great 

 perfection ; and also furnish us with examples of crystalline 

 change of very varying character, A most careful examination 

 of them, both in the field, microscopically, chemically, and by 

 means of the polariscope, has convinced me that the structure 

 on which their slaty cleavage depends may be completely ex- 

 plained on mechanical principles ; and that, instead of chemical 

 forces having given rise to it, they have had a contrary tendency; 

 for whilst, other things being the same, the perfection of cleavage 

 varies directly as the mechanical changes, it varies invei-sely as 

 the chemical. 



Before entering into the consideration of the peculiarities of 

 structure which give rise to the cleavage, it is desirable that I 

 should describe the actual physical and chemical constitution of 

 the rocks themselves. To study the real nature of limestones, it is 

 requisite to prepare sections of them that may be satisfactorily 

 examined with high powers of the microscope, for which purpose 

 they should be somewhere about the thousandth part of an inch 

 * Communicated by the Author. 



