326 Biographical Account of [May, 



Dr. Black's speculations respecting heat have had such an effect 

 upon the progress of the science, that it would be unpardonable not 

 to notice them here. A very good account of them will be found 

 in the first volume of his lectures. Indeed, his lectures on heat 

 Constituted the most finished, and by far the most valuable, part of 

 his course. It is well known that the freezing point of water is 32°^ 

 that whenever the thermometer sinks below 32° water begins to 

 freeze, and whenever it rises above 32° ice and snow begin to melt. 

 At the first view of the subject one would be disposed to expect that 

 as soon as the thermometer sinks below 32° the whole water would 

 immediately become ice, and that when it rises above 32** the ice 

 would be as speedily converted into water ; but every body knows 

 that these speedy changes never take place. In cold weather a crust 

 of ice is formed upon the surface of rivers and lakes ; and if the 

 cold continue, this crust becomes gradually thicker. But unless the 

 water be very shallow, it is very seldom or never totally converted 

 into ice. The warm weather returns while a considerable portion 

 of the water of the lake is still unfrozen. We remark the same 

 slowness in the conversion of ice into water. When snow is accu- 

 mulated in great quantities in mountainous countries, it resists the 

 united action of the sun and the wind for weeks, or even months. 

 It is always melting, indeed, but it melts very slowly; and in some 

 cases the cold weather returns again before the liquefaction is com- 

 pleted. Such were the facts which had been obvious to all the 

 world from the beginning. Dr. Black was the first person who 

 examined them closely and endeavoured to explain them. Accord- 

 ing to him, water is a compound of two substances — ice and heat. 

 It cannot freeze or be converted into ice till it has parted with its 

 heat ; and as the heat makes its escape but slowly, the water freezes 

 but slowly. Ice, on the other hand, can only be converted into 

 water by combining with a certain quantity of heat ; and as this 

 combination takes place but slowly, the ice melts but slowly. This 

 view of the subject Dr. Black confirmed by simple but satisfactory 

 experiments. The beat which thus renders water fluid he called 

 latent heat, because its presence is not indicated by the thermo- 

 meter. He showed that the latent heat of water is 140°. He 

 ascertained likewise that fluidity in all cases is owing to the combi- 

 nation of latent heat with the body becoming fluid. 



It is well known that water and other liquids, when exposed to 

 heat, increase in temperature till they become boiling hot, but after 

 that their temperature remains stationary. They gradually indeed 

 boil away, and are converted into steam or vapour, an elastic fluid 

 possessing many of the properties of air ; with this difference, that 

 when exposed to the action of cold it is again converted into the 

 very liquid from which it was originally produced. Dr. Black 

 showed that vapour or steam is a combination of the liquid from 

 which it wi.s produced and latent heat. The latent heat of the 

 vapour of water or steam he found about 950°. It was this doctrine 



