1863.] 549 



pressure causes a slower, and in others a quicker chemical action, 

 whilst in others it has scarcely any influence whatever ; and though, 

 for reasons already explained, I say it with some hesitation, yet, 

 bearing in mind what is already known respecting the action of 

 pressure on hydrate of chlorine, hydrated hydrosulphuric acid, and 

 other substances described by the various authors referred to in the 

 notes, I think the facts I have described make it very probable 

 that further research will show that pressure weakens or strengthens 

 chemical affinity according as it acts against or in favour of 

 the change in volume ; as if chemical action were directly con- 

 vertible into mechanical force, or mechanical force into chemical 

 action, in definite equivalents, according to well-defined general laws, 

 without its being necessary that they should be connected by means 

 of heat or electricity. On the present occasion I shall not attempt 

 to consider the various geological and mineralogical facts which 

 appear to me to admit of the application of the principles I have 

 described, for many of them are peculiarities in structure of which 

 neither myself nor any one else has ever given a description, and would 

 therefore demand a preliminary notice. However, I may say that it 

 appears to me that a number of facts connected with metamorphic 

 rocks and the phenomena of slaty cleavage, which, to me at all 

 events, have hitherto been inexplicable, are readily explained if 

 mechanical force be directly correlated to chemical action, and if in 

 some cases the direction in which crystals are formed be more or less 

 related to pressure, in some such way as there is a connexion between 

 their structure and magnetic force, as shown by the experiments of 

 Pliicker, Faraday, Tyndall, and many other observers. We may 

 also, I think, explain the origin of the impressions on the limestone 

 pebbles in the " Nagelflue " in Switzerland, about which so much 

 has tyeen written in Germany and 'France, without a satisfactory 

 reason having been discovered ; and the same explanation accounts 

 for the mutual penetration of the fragments of which some limestones 

 are formed, a,nd for the banded structure of some which possess 

 slaty cleavage. The curious teeth-like projections with which one 

 bed of limestone sometimes enters into another, also to a certain 

 extent indicate a chemical action depending on mechanical force ; 

 and probably the same may be said of some of the peculiarities of 

 slickensides and mineral veins. It is also possible that a pressure 



VOL. XII. 2 R 



