it is withdrawn, the thermometer immersed in the mercury, instead of remaining fixed at 200°, will begin to rise, although the action of the lamp remains the same as before; from which it is evident that the heat which now causes the mercury to rise above 200° was before received by the melting

ice.

The heat which thus enters ice in the process of liquefaction, and which is not indicated by the thermometer, is for this reason called latent heat. It will be perceived that this phrase is the name of a fact, and not of an hypothesis. That heat really enters the water, and is contained in it, has been established by the experiments; and to declare that it is present there, is to declare an established fact. To call it by the name latent heat, is to declare another established fact, viz., that it is not sensible to the thermometer.

These facts show us that heat is capable of existing in bodies in two distinct states, in one of which it is sensible to the thermometer, and in the other not. Heat which is sensible to the thermometer is called, for distinction, sensible or free heat. It may be here observed, that heat which is sensible to the thermometer is also perceptible by the senses, and heat not sensible to the thermometer is not perceptible by the senses. Thus, ice at 32° and water at 32° feel equally cold, and yet we have seen that the latter contains considerably more heat than the former.

Dr. Black, who first noticed the remarkable fact to which we have now alluded, inferred that ice is converted into water by communicating to it a certain quantity or dose of heat, which enters into combination with it in a manner analogous to that which takes place when bodies combine chemically. The heat, thus combined with the solid ice, loses its property of affecting the senses or the thermometer, and the effects therefore bear a resemblance to those cases of chemical combination in which the constituent elements change their sensible properties when they form the compound.

The fact that the thermometer immersed in the ice remains stationary only as long as the process of liquefaction is going on, shows that this absorption of heat is necessarily connected with that process, and that, were it not for the conversion of

the solid ice into liquid water, the heat which is so received would be sensible, and would cause the thermometer immersed in the ice to rise. Before the time of Black it was supposed that the slightest addition of heat would cause solid ice to be converted into water, and that the thermometer would immediately pass from the freezing temperature to higher degrees. The experiments above described, however, show the falsehood of such a supposition. If, while the mercurial bath, in which the ice is immersed, is maintained at the temperature of 200°, the length of time necessary to complete the liquefaction of the ice be observed, it would be found that that time is about twenty-eight times the length of time which it would take to raise the liquid water from 32° to 37°; and if it be assumed that the same quantity of heat is imparted to the ice, during the process of liquefaction, during each minute, as is imparted to the water, during each minute, in rising from 32° to 37°, it will follow, that to liquefy the ice requires twentyeight times as much heat as is necessary to raise the water from 32° to 37°. It appears, therefore, that, instead of a small quantity of heat being necessary to melt the ice, a very considerable portion is absorbed in that process.

Having ascertained the remarkable fact, that heat is absorbed in a large quantity in the conversion of ice into water, without rendering the body so absorbing it warmer, let us now inquire what the exact quantity of heat so absorbed is. We have already stated that, if the quantity communicated in equal times be the same, the heat necessary to liquefy a given weight of ice would be twenty-eight times as much as would be necessary to raise the same weight of water from 32° to 37°; or, if the heat necessary to raise water through every 5° be the same, that quantity of heat would be sufficient to raise water from 32° to 172°: and hence we infer, that as much heat is absorbed in the liquefaction of a given quantity of ice as would raise the same quantity of water through 140 degrees of the thermometric scale.

(52.) Let us now examine the analogous effects produced by the continued application of heat to water in the liquid state.

Let a small quantity of water be placed in a glass flask of considerable size, and then closed so as to prevent the escape

of any vapour. Let this vessel be now placed over the flame of a spirit lamp, so as to cause the water it contains to boil. For a considerable time the water will be observed to boil, and apparently to diminish in quantity, until at length all the water disappears, and the vessel is apparently empty. If the vessel be now removed from the lamp, and suspended in a cool atmosphere, the whole of the interior of its surface will presently appear to be covered with a dewy moisture; and at length a quantity of water will collect in the bottom of it, equal to that which had been in it at the commencement of the process. That no water has at any period of the experiment escaped from it, may be easily determined, by performing the experiment with the glass flask suspended from the arm of a balance, counterpoised by a sufficient weight suspended from the other arm. The equilibrium will be preserved throughout, and the vessel will be found to have the same weight, when to all appearance it is empty, as when it contains the liquid water. It is evident, therefore, that the water exists in the vessel in every stage of the process, but that it becomes invisible when the process of boiling has continued for a certain length of time; and it may be shown that it will continue to be invisible, provided the flask be exposed to a temperature considerably elevated. Thus, for example, if it be suspended in a vessel of boiling water, the water which it contains will continue to be invisible; but the moment it is withdrawn from the boiling water, and exposed to the cold air, the water will again become visible, as above mentioned, forming a dew on the inner surface, and finally collecting in the bottom, as in the commencement of the experiment.

In fact, the liquid has, by the process of boiling, been converted into vapour, or steam, which is a body similar in its leading properties to common air, and, like it, is invisible. It will hereafter appear that it likewise possesses the property of elasticity, and other mechanical qualities enjoyed by gases in general.

(53.) Again, let an open vessel be filled with water at 60°, and placed in a mercurial bath, which is maintained, by a fire or lamp applied to it, at the temperature of 230°. Place a thermometer in the water, and it will be observed gradually to

rise as the temperature of the water is increased by the heat which it receives from the mercury in which it is immersed. The water will steadily rise in this manner until it attains the temperature of 212°; but here the thermometer immersed in it will become stationary. At the same time the water contained in the vessel will become agitated, and its surface will present the same appearance as if bubbles of air were rising from the bottom, and issuing at the top. A cloudy vapour will be given off in large quantities from its surface. This process is called ebullition or boiling. If it be continued for any considerable time, the quantity of water in the vessel will be sensibly diminished; and at length every particle of it will disappear, and the vessel will remain empty. During the whole of this process, the thermometer immersed in the water will remain stationary at 212°.

Now, it will be asked, what has become of the water? It cannot be imagined that it has been annihilated. We shall be able to answer this by adopting means to prevent the escape of any particle of matter from the vessel containing the water, into the atmosphere or elsewhere. Let us suppose that the top of the vessel containing the water is closed, with the exception of a neck communicating with a tube, and let that tube be carried into another close vessel removed from the cistern of heated mercury, and plunged in another cistern of cold water. Such an apparatus is represented in fig. 15.

A is a cistern of heated mercury, in which the glass vessel B, containing water, is immersed. From the top of the vessel B proceeds a glass tube c, inclining downwards, and entering a glass vessel D, which is immersed in a cistern E of cold water. If the process already described be continued until the water by constant ebullition has disappeared, as already mentioned,

from the vessel B, it will be found that a quantity of water will be collected in the vessel D; and if this water be weighed, it will be found to have exactly the same weight as the water had which was originally placed in the vessel B. It is, therefore, quite apparent that the water has passed by the process of boiling from the one vessel to the other; but, in its passage, it was not perceptible by the sight. The tube c and the upper part of the vessel B, had the same appearance, exactly, as if they had been filled with atmospheric air. That they are not merely filled with atmospheric air may, however, be easily proved. When the process of boiling first commences, it will be found that the tube c is cold, and the inner surface dry. When the process of ebullition has continued a short time, the tube c will become gradually heated, and the inner surface of it covered with moisture. After a time, however, this moisture disappears, and the tube attains the temperature 212°. In this state it continues until the whole of the water is discharged from the vessel B to the vessel D.

(54.) These effects are easily explained. The water in the vessel B is incapable of receiving any higher temperature than 212°, consistently with its retaining the liquid form. Small portions, therefore, are constantly converted into steam by the heat received from the surrounding mercury, and bubbles of steam are formed on the bottom and sides of the vessel B. These bubbles, being very much lighter, bulk for bulk, than water, rise rapidly through the water, just in the same manner as bubbles of air would, and produce that peculiar agitation at its surface which has been taken as the external indication of boiling. They escape from the surface, and collect in the upper part of the vessel. The steam thus collected, when it first enters the tube c, is cooled below the temperature of 212° by the surface of the tube; and consequently, being incapable of remaining in the state of vapour at any lower temperature than 212°, it is reconverted into water, and forms the dewy moisture which is observed in the commencement of the process on the interior of the tube c. At length, however, the whole of the tube c is heated to the temperature of 212°,