If, indeed, there were any thing, in the mere appearance of a body, which could enable us to predict the changes that would take place in it, when brought into every possible variety of situation, with respect to other bodies, or the changes which it would then produce in those other bodies, the two views, into which I have divided physical inquiry, would coincide exactly; so that to know the _continuous elements_ of any substance, would be to know, at the same time, its _susceptibilities_ and _powers_. But there is nothing, in the mere sensible qualities of bodies, considered separately, that can give us even the slightest intimation of the changes, which, _in new circumstances of union_, they might reciprocally suffer or produce. Who could infer, from the _similar_ appearance of a lump of sugar and a lump of calcareous spar, that the one would be soluble in water, and the other remain unmelted; or, from the _different_ aspect of gunpowder and snow, that a spark would be extinguished, if it fell upon the one, and, if it fell upon the other, would excite an explosion that would be almost irresistable? But for experience, we should be altogether incapable of predicting any such effects, from either of the objects compared; or, if we did know, that the peculiar susceptibility belonged to one of the two, and not the other, we might as readily suppose, that calcareous spar would melt in water as sugar, and as readily, that _snow_ as that _gunpowder_ would detonate, by the contact of a spark. It is _experience_ alone, which teaches us that these effects ever take place, and that they take place, not in all substances, but only in some particular substances.