It is a characteristic of great genius to find important matter of reflection in objects which the vulgar pass by as too common to excite notice, and Dr. Black having remarked some very common facts with regard to heat, was conducted to those great discoveries on which his celebrity rests:—that of _latent heat_, and that of _specific heat_; which last term is, in fact, only another mode of expressing the same principle. This great truth, the foundation of all our determinate ideas of the causes of those diversities of physical condition which the same mass of matter is capable of assuming, seems to have suggested itself to the mind of the discoverer about the year 1757. After the invention of the thermometer, it had been among the earliest facts observed that changes in the state of bodies, such as boiling, freezing, melting, &c., take place always at certain fixed temperatures as indicated by the thermometer; and at a different degree of the scale, for each different substance. And several of these remarkable points came by custom to be marked upon the thermometric scale. When however it was said that water always boiled at 212° of Fahrenheit, or froze at 32°, &c., it was not meant that the mass would boil or freeze the instant the thermometer reached that point. It was supposed that a certain increase or diminution of temperature (as the case might be) was necessary for the production of the effect beyond that precise point; though that point marked, as it were, the commencement of the process. The views generally entertained on this subject were however so vague, that it is difficult to make out precisely what was imagined to take place; but it seems to have been supposed, that a very slight accession or loss of heat was sufficient completely to accomplish the change.