546 [April 30, 



pressure would be very different, as is confirmed by experiment in 

 the case of these and some other salts. 



The facts I have described, therefore, show that there is a direct 

 correlation between mechanical force and the forces of crystallization 

 and solution. According to some chemists, the latter is an instance 

 of real combination ; but, whatever views be entertained respecting 

 its nature, we cannot, I think, deny that the force represents some 

 modification of chemical affinity, or is at all events most closely 

 allied to it. In comparison with some kinds of affinity, it may 

 indeed be, and probably is, weak ; but yet, as I have shown, it 

 sometimes has a very considerable mechanical equivalent, even when 

 nearly counterbalanced by an opposite force ; and since such pres- 

 sures as glass tubes will resist have no very great influence on what 

 we may perhaps consider a weak affinity, we cannot expect that any 

 pressure at our command would have much influence on strong 

 affinities. I have, however, succeeded in obtaining some results 

 which apparently show that pressure influences undoubtedly chemical 

 changes taking place slowly, and therefore probably due to weak, or 

 nearly counterbalanced, affinities. 



The method adopted in this part of the inquiry was to seal up 

 some solid substance in a solution which gives rise to a slow double 

 decomposition, taking great care to have in the tube with pressure, 

 and in that without, pieces cut so as to be of the same size and form, 

 and a solution of the same character, so that, with the exception of 

 pressure, all the conditions were the same. Possibly I may be so 

 fortunate as to discover some case where the affinity is so weak 

 that pressure may determine whether it go forward or not, of which 

 fact the structure of metamorphic rocks furnishes examples ; but 

 hitherto I have only been able to prove that pressure modifies the 

 rate at which chemical action takes place. This branch of the 

 inquiry is, however, beset with many difficulties, for the change in 

 volume produced by double decomposition is small, and its deter- 

 mination involves several complicated questions. The volume of the 

 solids is easily determined ; but that of the salts in solution is not the 

 same when other salts are present as when they are dissolved in pure 

 water, and varies much according to the strength of the solution and 

 the nature of the salts ; and many points are still so obscure, that I 

 shall only give two cases by way of example. 



