475 



presented a perfect impression of the tool marks on the bottom of the 

 piston, such as might have been made in wax. The expulsion of the 

 minute quantities of brine remaining in pores in the salt when it has 

 become very closely compacted, appears to be a slow and difficult 

 process ; as, after the pressure had been continued for about a fort- 

 night, I still found a slight oozing of brine from a pore which hap- 

 pened to exist in the side of the cylinder. 



Experiments by the application of tensile stresses, or of any other 

 stresses than those mixed and chiefly compressive ones which arise 

 when the crystals are pressed in a close vessel by a rammer, would 

 probably not be very easily carried out ; and I have not as yet tried 

 any except those by pressure. I feel quite convinced, however, that 

 melting, or dissolving, must result from all kinds of stresses tending 

 to change of form. I think the following statement may be assumed 

 as a general physico-mechanical principle or axiom, and I think it 

 involves the truth of the opinion just expressed : 



If any substance, or any system of substances, be in a condition 

 in which it is free to change its state (whether of molecular arrange- 

 ment, or of mechanical relative position and connexion of its parts, 

 or of rest or motion), and if mechanical work be applied to it (or put 

 into it) as potential energy, in such a way as that the occurrence of 

 the change of state will make it lose (or enable it to lose) (or be ac- 

 companied by its losing) that mechanical work from the condition of 

 potential energy, without receiving other potential energy as an equi- 

 valent ; then the substance or system will pass into the changed 

 state. The consideration of a few cases, in some of which there is 

 not freedom for the substance or system to change its state, and in 

 others of which there is freedom, will render the meaning of this 

 more clear. 



Gunpowder may be cited as an example of a substance in a condi- 

 tion not free to change its state, although when it is made to explode 

 by a spark, it passes to an altered condition, and, in doing so, even 

 gives out a great amount of mechanical work. That is to say, that 

 on the whole it is more than free to change to the exploded state, or 

 it tends so to change, but there is some kind of obstacle at ordinary 

 temperatures, to the change, which either vanishes at a high tempera- 

 ture, or requires the application of mechanical work to begin the over- 



