44 REPORT—1858. 
quences,—so entire a system of nature is to be based on a principle,—nothing short of 
such evidence ought, I think, to be held conclusive, however seductive the theory 
may appear. I do not think such precision unattainable, and I think I perceive a 
way in which it might be attained, but one that would involve an expenditure of 
time, labour, and money, such as no private individual could bestow on it. 
If the phenomena of chemistry are ever destined to be reduced under the domi- 
nion of mathematical analysis, it will, no doubt, be by a very circuitous and intricate 
route, and in which at present we see no glimpse of light. We should, therefore, be 
all the more carefully on the watch in making the most of those classes of facts, which 
seem to place us, not indeed within view of daylight, but at what seems an opening 
that may possibly lead to it. Such are those in which the agency of light is concerned 
in modifying or subverting the ordinary affinities of material elements, those to which 
the name of actino-chemistry has been affixed. Hitherto the more attractive appli- 
cations of photography have had too much the effect of distracting the attention from 
the purely chemical questions which it raises ; but the more we consider them in the 
abstract, the more strongly they force themselves on our notice; and I look forward 
to their occupying a much larger space in the domain of chemical inquiry than is the 
case at present. That light consists in the undulations of an etherial medium, or at 
all events agrees better in the characters of its phenomena with such undulations, than 
with any other kind of motion which it has yet been possible to imagine, is a pro- 
position on which I suppose the minds of physicists are pretty well made up. The 
recent researches of Prof. Thomson and Mr. Joule, moreover, have gonea great way 
towards bringing into vogue, if not yet fully into acceptation, the doctrine of a more 
or less analogous conception of heat. When we consider now the marked influence 
which the different calorific states of bodies have on their affinities—the change of 
crystalline form effected in some by achange in temperature,—the allotropic states taken 
on by some on exposure to heat, —or the heat given out by others on their restoration 
from the allotropic to the ordinary form (for though I am aware that Mr. Gore considers 
his electro-deposited antimony to be a compound, I cannot help fancying that at all 
events the state in which the antimony exists in it is an allotropic one),—when, I say, 
we consider these facts in which heat is concerned, and compare them with the facts 
of photography, with the ozonization of oxygen by the chemical rays of the electric 
spark, and with the striking alterations in the chemical habitudes of bodies pointed 
out by Draper, Hunt, and Becquerel; and when, again, we find these carried so far, 
that, as in the experiments of Bunsen and Roscoe, the amount of chemical action 
numerically measures the quantity of light absorbed,—it seems hardly possible not to 
indulge a hope that the pursuit of these strange phenomena may by degrees conduct 
us to a mechanical theory of chemical action itself. Even should this hope remain 
unrealized, the field itself is too wide to remain unexplored; and to say nothing of 
discovery, the use of photography merely as a chemical test may prove very valuable, 
as I have myself quite recently experienced, in the evidence it has afforded me of the 
presence in certain solutions of a peculiar metal having many of the characters of 
arsenic, but differing from it in others, and strikingly contrasted with it in its powerful 
photographic qualities, which are of singular intensity, surpassing iodine, and almost 
equalling bromine. 
There is another class of phenomena, which, though usually considered as belonging 
peculiarly to the domain of general physics, and so out of our department, seems to 
me to want some attention in a chemical point of view ;—it is that of capillary attrac- 
tion. The coefficient of capillarity differs very remarkably in different liquids, and 
no doubt also in their contacts with different solids;—a fact, which can hardly be 
separated from the idea of some community of nature between the capillary force and 
those of elective attraction. I hardly dare to hint at the existence of some slight 
misgiving I have always felt as to the validity of the received statical theory of capil- 
lary action, which carries with it the authority of such names as those of Laplace and 
Poisson. Any discussion of this point would be matter for another Section of this 
Association ; and if I here touch upon it, it is only to observe that my impression of 
the requisiteness of a force so far allied to chemical affinily as to be capable of satura- 
tion, rests on other grounds besides that of the mere diversity of action above alluded 
to. But I must remember that you are not met here to listen to generalities, of what- 
ever nature, but that we have plenty of real and special business before us. 
