peculiarities as this is supposed to imply, are now the only _differentiæ_ which form the fixed connotation of the word Acid, as a term of chemical science. Scientific men are still seeking, and may be long ere they find, a suitable definition of one of the earliest words in the vocabulary of the human race, and one of those of which the popular sense is plainest and best understood. The word I mean is Heat; and the source of the difficulty is the imperfect state of our scientific knowledge, which has shown to us multitudes of phenomena certainly connected with the same power which causes what our senses recognise as heat, but has not yet taught us the laws of those phenomena with sufficient accuracy to admit of our determining under what characteristics the whole of those phenomena shall ultimately be embodied as a class: which characteristics would of course be so many differentiæ for the definition of the power itself. We have advanced far enough to know that one of the attributes connoted must be that of operating as a repulsive force; but this is certainly not all which must ultimately be included in the scientific definition of heat. What is true of the definition of any term of science, is of course true of the definition of a science itself: and accordingly, (as observed in the Introductory Chapter of this work,) the definition of a science must necessarily be progressive and provisional. Any extension of knowledge or alteration in the current opinions respecting the subject matter, may lead to a change more or less extensive in the particulars included in the science; and its composition being thus altered, it may easily happen that a different set of characteristics will be found better adapted as differentiæ for defining its name.