1887.] Capillary Action in some Chemical Decompositions. 73 
comparable in order of magnitude, and I do not mean that it 
shews anything more. | 
The difference between the surface energy of water in contact 
with mercury, and of a solution of hydrochloric acid in contact 
with mercury, is nearly twelve times as great as the difference 
which I have taken above for my illustration, so that I have not 
taken an exceptionally favourable case. 
Charcoal is by no means a pure chemical substance, and small 
differences in its composition according as it is made from animal 
or from vegetable tissues may well give it different surface tensions 
when in contact with the same bodies. Such different surface 
tensions will, I think, suffice to account for the different activities 
of animal and vegetable charcoal when used as filters. 
I have not attempted to trace the influence of capillarity in the 
chemical changes induced in substances thus adherent to charcoal, 
vegetable mould, and other filtering materials, though it seems 
probable that it may play an important part in bringing about 
such changes. It may assist materially in the processes of secre- 
tion, and in the rapid exchange of oxygen for carbonic acid gas 
which occurs in respiration; and it can hardly fail to have its 
influence in many of the chemical changes which occur in the 
organs of plants and animals. 
Further, it does not appear to be in any way necessary that the 
mutually reacting bodies should either of them be a fluid. For 
solids have their surface energies and must influence one another 
when sufficiently approached. In this way we may explain the 
action of manganese dioxide in promoting the disengagement of 
oxygen from potassium chlorate at temperatures below the fusing 
point of the latter. 
That such disengagement does occur before the potassium 
chlorate enters into fusion I take on the authority of Wiederholt 
and others, though in general, so far as my own observations go, 
the decomposition of the chlorate is always accompanied by fusion 
of the part decomposed. 
That solution is the spreading of a film of the dissolved sub- 
stance over the internal surface of the menstruum has been, I 
believe, suggested before. No one has as yet attempted to work 
out the evidence for or against such a supposition. ‘The problem 
however is a very interesting one, as it is well known that the 
solution of substances, and their separation from solution, have 
considerable influence upon chemical changes. The supposition 
seems to offer an obvious explanation of the deposition of crystals 
upon nuclei and upon surfaces from which the adhering films have 
been partly removed by rubbing. 
