From whatever source caloric is obtained, it passes from bodies in which it is accumulated in a free state into bodies which contain less of it, until both are brought to an equilibrium. "The state of a body with regard to its power of producing the different effects arising from the presence of caloric, is termed its temperature;" and this depends on the quantity of sensible caloric contained in it. Thus, when a vessel containing water is placed on the fire, a quantity of caloric passing from the fire into the water, the temperature of the water is raised, or it is made sensibly hotter; and if the water, thus heated, be taken from the fire and placed in a cold place, the sensible caloric accumulated in it passes from it into the air and surrounding bodies, until it become as cold as they are, or until its temperature be lowered to an equilibrium with theirs. The caloric lost by hot bodies during their cooling is carried off: 1st, by the conducting power of the surrounding medium, which "diminishes as the tern-perature of the hot body approaches to that of the medium;" 2dly, by radiation; 3dly, by currents, or the repeated changes of the portion of medium immediately in contact with the hot body, produced by the change of density, occasioned by the caloric they receive from the hot body, enabling them to rise and give place to a new portion, which, being heated, is also displaced in its turn, and so on till the temperature of the hot body approaches to that of the medium.

By accelerating these changes, the rate of cooling is propor-tionably quickened; and thence the cooling effect of winds and artificial currents of air.

The temperature of bodies can be comparatively ascertained, to a certain extent, by the sensations they induce. Thus, a body containing much sensible caloric feels warm or hot to the touch, owing to its caloric flowing into the hand; and one containing less than the human body gives the sensation of cold, owing to the abstraction of caloric from the hand. But this mode of judging of temperature is very limited, and depends on the state of the sentient organ, the conducting power of the body which is touched, and many other external circumstances, which prevent confidence from being placed on it as a comparative measure of temperature; and, therefore, instruments have been invented to measure the degrees of temperature of different bodies, the properties of which depend on the expansion or increase of bulk which bodies suffer when caloric enters into them.

The thermometer is a most useful and important instru merit of this kind. It is a hollow glass tube, having at one end a hollow globe or bulb, the bore of the tube being perfectly cylindrical and small, and the bulb of a proportional size. The bulb, and a portion of the tube, after the air is expelled by holding the bulb over the flame of a lamp, are filled with mercury or coloured alcohol, by immersing the open end of the tube in either of these fluids: as the air which still remains in the tube cools, the mercury is forced up into the tube, and supplies the place of the air which heating it had expelled. The remaining air is expelled by boiling the mercury, and the tube is then hermetically sealed at the extremity. When the bulb of this instrument is applied to a hot body, the mercury, or the fluid it contains, rises in the tube, and continues to do so until the thermometer acquires the same degree of temperature as the hot body, when the mercury becomes stationary, and the point to which it rises indicates the relative temperature of the hot body. In the same manner, when the bulb is applied to a cold body, the mercury contracts and falls in the tube, owing to the abstraction of caloric by the cold body.

The height to which this rises or falls, indicating the proportion of increase or the diminution of temperature, is ascertained by a scale, which divides the tube into a number of equal parts or degrees.1 As the instrument may be occasionally plunged into corrosive fluids, part of the stem (a) and the bulb (b) should project beyond the scale; or the scale may turn up with a hinge. A thermometer has been made by the late Mr. Jowitt, one of my pupils, which encloses the scale in a glass-case affixed to the thermometer in every part, except the bulb, which permits it to be used in the most active media.