146 
reacts, as has been set forth here for the action of chlorine, we 
shall have to assume that the transformations to which a metal is 
subjected, practically take exclusively place at the surface of the 
solid metal. Thus e.g. the generation of hydrogen, the deposition of 
a nobler metal by a less noble one ete. Considerations like those 
that have been applied in $ 3 of the preceding papers, have also 
been frequently applied for other transformations, not however for the 
purpose of showing how the reaction in reality proceeds for the 
greater part, but to indicate im what direction the transformation must 
take place, and what final state is reached. 
We remind e.g. of the transformation of one modification of a sub- 
stance into another. The most stable form having the smallest vapour 
tension and the smallest solubility, it is clear that the metastable 
form must be transformed into the more stable form in contact with 
vapour or solution. This, however, does not mean that this trans- 
formation always takes place chiefly over the vapour or the solu- 
tion, on the contrary it may be predicted with certainty in virtue 
of what was derived above, that these transformations, when they 
take place quickly, do not take place via the vapour or the solu- 
tion; this can only take place when the transformation takes place 
exceedingly slowly. 
Also for the dissolving of a deposit by the addition of reagents 
generally the conception is used that the deposition goes into solu- 
tion, and is then converted in the aqueous solution by the added 
substance. The solution of CaCO, into HCI e.g. is represented as follows: 
Ca CO ne [Ca CO, Ca + CO," IL 
(CO,"+ 2H 24H,C0O,2 7,0 + CO,)1 
and the solution of AgC/in NH, by the equations: 
Ag Cls @lAg Cla Ag + Has 
[Ag + 2NH, 2 Ag (AH). Iz 
The equations, however, only serve to show that CaCO, dissolves 
in HCI and AgCl in NH,, and what conception can be used then 
in connection with the equilibria that can occur in the said system. | 
But it does not follow at all from this that CaCO, really chiefly 
goes into solution in this way. In connection with the slight solu- 
bility of the substance in water this is certainly by no means the 
case, and the solution must therefore take place by the action of 
HCI or H' on the solid CaCO,, and of NH, on the solid AgCl. 
The same objection that a reaction would proceed quickly with a 
very small concentration of one of the reacting components has 
already been discussed with the electrolytic deposition of metal from 
