In my last Lecture, Gentlemen, I considered, at some length, the nature of _Physical Inquiry in general_, and stated to you, in particular, the _two lights_, in which objects may be physically viewed, as existing simply in _space_, or as existing in _time_,--the inquiries, with respect to the one, having regard to the composition of bodies; the inquiries, with respect to the other, having regard to the changes, of which they are either the subjects or occasions, and consequently to their susceptibilities or their powers--their susceptibilities of being affected by other substances, their powers of affecting other substances. I use the word _susceptibility_, you will perceive, as, in this case, synonymous with what Mr Locke, and some other writers, have denominated _passive power_, to avoid the apparent verbal contradiction, or at least the ambiguity, which may arise from annexing the term _passive_ to a word, which is generally employed to signify, not the _subject_ of change, but the cause or occasion of change. Of these two points of view, then, in which an object may be regarded, when the question is put, What is it? we have seen, I hope, sufficiently distinctly, the nature of one. If, in answering the question, we regard the object merely as it exists in space, and say, that it is a _compound_ of certain substances, we mean nothing more, than that, in the portion of space, which we conceive to be occupied by this one imaginary aggregate, there is truly a plurality of bodies, which, though seemingly contiguous, have an existence, as separate and independent of each other, as if they were at the most remote distance; the one aggregate being nothing more than a name for these separate bodies, to which ourselves give all the unity which they have, merely by considering them as _one_. The necessity of inquiring into the nature of these separate elementary bodies,--which constitutes one of the two great departments of physical investigation,--we found to arise from the imperfection of our senses, that are not sufficiently acute to discover, of themselves, the component parts of the masses, which nature everywhere presents to us. We are thus obliged to form to ourselves an _art of analysis_, merely that we may perceive what is constantly before our eyes, in the same manner, as we are obliged to have recourse to the contrivances of the optician, to perceive stars and planets, that are incessantly shedding on us their light.