In all chemical operations, as already observed, we only witness the beginning and the end; the cause and the effect; while the whole of the intermediate changes elude our senses. Nevertheless, by a careful observation of the phenomena we are enabled to form some notion of these operations; and that amply sufficient to convince us of their wonderful nature. With a view therefore of arresting the attention of readers who may be unconscious of these wonders, or too apt to overlook them; we have thought it proper to premise a sketch of what may be supposed to take place, among the ultimate particles of which all bodies are constituted; during those remarkable changes which they are constantly undergoing. And here it may be remarked, once for all, that many of the views commonly entertained on these points, have always appeared to us to be so imperfect and unsatisfactory; that so far from elucidating the subject, they have only served to render it the more obscure. In the following sketch, therefore, as better adapted for our purpose, we have endeavoured to give that view of the subject, which, after twenty years of close attention and no ordinary labour, we have been induced to consider as the most simple and consistent with the phenomena. The general reader, who feels no interest in such enquiries, but who at the same time wishes to be apprized of the nature of the arguments deducible from the divisibility and molecular constitution of matter, is referred to the end of the present, and of the following chapters, for a summary of these arguments.