674 BRITISH ASSOCIATION FOR ADVANCEMENT OF SCIENCE. 
in the highest degree characteristic; and what is more remarkable, the 
initial member in each group possessing in every case the characteristic 
property of the group in its most eminent degree, while the others 
exhibit that property in a less and less degree, according to their rank 
in the progression, or according to the increased numerical value of the 
atomic equivalent. Generally speaking, I am a little slow to give full 
credence to numerical generalizations of this sort, because we are apt 
to find their authors either taking some liberties with the numbers them- 
selves, or demanding a wider margin of error in the application of their 
principles, than the precision of the experimental data renders it 
possible to accord, so that the result is more or less wanting in that close 
appliance to nature which makes all the difference between a loose 
analogy and a physical law; but in this instance it certainly does appear 
that the groups so arising not only do correspond remarkably well in 
their theoretical numbers with those which the best authorities assign 
to their elements, but that it really would be difficult to distinguish the 
elements themselves into more distinctly characteristic classes by a con- 
sideration of their qualities alone, without reference to their atomic 
numbers. When we find, for instance, that the principle affords us 
such family groups as oxygen, fluorine, chlorine, bromine, and iodine 
self-arranged in that very order ; or again, nitrogen, phosphorus, arsenic, 
antimony, or bismuth ; when we find that it packs together in one group 
all the more active and soluble electro-positive elements, hydrogen, 
lithium, sodium, and potassium, and in another the more inert and less 
soluble ones, calcium, strontium, barium, and lead — and that without 
outraging any other system of relations, it certainly does seem that we 
have here something very like a valid generalization ; and I shall be 
very glad to learn in the course of any discussions which may arise on 
such matters as may be brought before us in the regular conduct of our 
business from those more competent to judge than myself, whether I 
have been forming an overweening estimate of the value and importance 
of such generalizations.” 
The Phenomena of Chemistry, and the Nature and Effects 
of Light and Heat. 
“ If the phenomena of chemistry are ever destined to be reduced under 
the dominion 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 con- 
cerned 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 applications of photography have had too 
much the effect of distracting the attention from the purely chemical 
question 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 che- 
mical inquiry than is the case at present. That light consists in the 
undulations of an ethereal 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 proposi- 
tion on which I suppose the minds of physicists are pretty well made 
up. The recent researches of Professor Thomson and Mr. Joule more- 
over have gone a great way towards bringing into vogue, if not yet fully 
