Page:Biographia Hibernica volume 1.djvu/119

 108 BLACK spicuity would be improper. The axioms, however, which he established, were usually expressed by him in the following terms:_ 1. When a solid body is converted into a fluid, there enters into it, and unites with it, a quantity of heat, the presence of which is not indicated by the thermometer; and this combination is the cause of the fluidity which the body assumes. On the other hand, when a fluid body is converted into a solid, a quantity of heat separates from it, the presence of which was not formerly indicated by the thermometer ; and this separation is the cause of the solid form the fluid assumes. e. When a liquid body is raised to the boiling tempe- rature by the continued and copious application of heat, its particles suddenly attract to themselves a great quan- tity of heat, and by this combination their mutual relation is so changed, that they no longer attract each other, but are converted into an elastic fluid like ar. On the other band, when these elastie fluids, either by condensation or by the application of cold bodies, are re-converted into liquids, they give out a vast quantity of heat, the presence of which was not formerly indicated by the thermometer. Thus water, when it assumes the solid form, or is con- verted into ice, gives out 140° of heat; and ice, in becom- ing water, absorbs 140 of heat. Thus again, water in being converted into steam, absorbs about 1000 of heat, without becoming sensibly hotter than 212. The ther- mometer had long been considered by chemical philoso- phers as the only method of discovering the degree of heat in bodies; yet this instrument gives no indication of the presence of the 140 of heat which combine with ice to convert it into water, nor of the 1000 which combine with water when it is converted into steam. Dr. Black, therefore, said that the heat is concealed (latet) in the water and steam, and he briefly expressed this fact by ap- plying to the heat, in this case, the term of latent heat. It may, however, be necessary to observe, that though