(51.)

We shall pause here to put the reader in possession of the physical and mechanical principles connected with the evaporation of water and other liquids, which are necessary to enable him to understand the full extent of the value and the merit of the discoveries of Watt, and to comprehend the [Pg098] structure and operation of the steam engine in its improved form, as it has passed to us from his hands.

As we shall frequently have occasion to refer to the indications of a thermometer, we shall first explain the principle of that instrument as it is commonly used in this country.

The thermometer is an instrument used for the purpose of measuring and indicating the temperature or sensible heat of material substances.

Heat, like all other physical agents, can only be measured by its effects. One of these effects best suited for this purpose, is the change of dimension which all bodies undergo in consequence of their change of temperature. In general, when heat is applied to a material substance, that substance undergoes an enlargement of bulk; and if heat be abstracted from it, it suffers a diminution of bulk. This variation of magnitude is not always in the same proportion as the increase or diminution of temperature; but it is so when applied to certain substances and between certain limits. One of the substances whose expansion and contraction through an extensive range of temperature has been found to be nearly uniform, and which is attended with other convenient qualities for a thermometer, is the liquid called mercury or quicksilver. A mercurial thermometer is constructed in the following way:—

A glass tube is made with a small and uniform bore: upon the end of this tube, a bulb is blown, having a magnitude very great compared with the bore of the tube. Let us suppose this bulb and a part of the tube to be filled with mercury. If the mercury contained in the bulb be heated, it will expand, and being more susceptible of expansion than the glass which contains it, the bulb will be too small for its augmented volume: the mercury in the bulb can only, therefore, obtain room for its increased bulk by pressing the mercury in the tube upwards, which it will accordingly do. The increase of volume which the mercury in the bulb therefore undergoes, will be exhibited by the increased length of the column in the tube. Since the bore of the tube is made so exceedingly minute compared with the magnitude of the bulb, a very small quantity of mercury forced [Pg099] from the bulb into the tube, will cause a considerable increase of the length of the column. Small degrees of expansion will therefore be rendered very apparent, and may be accurately measured. The following is the method by which the thermometer called Fahrenheit's thermometer is graduated.

The tube and bulb being prepared and supplied with mercury, as already explained, let the instrument be plunged in a vessel of melting ice. It will be found that the mercury will stand in the tube at a certain point, from which it will not vary so long as any ice remains not completely melted in the vessel. Let a mark be made on the tube, or on a scale attached to the tube, at the point corresponding to the top of the column: the point thus marked is called the freezing point.

Now let the instrument be immersed in a vessel of boiling water, the barometer at the time having the height of thirty inches. It will be found that so long as the water is kept boiling, the column of mercury in the tube will remain stationary. Let the point corresponding with the top of the column be marked on the tube, or on the scale attached to it. This is called the boiling point. Let the space on the scale between the freezing and boiling points be now divided into 180 equal parts: each of these parts is called a degree. Let the same divisions be continued upon the scale below the freezing point, until thirty-two divisions be taken; let the lowest division be then marked 0, and let the successive divisions upwards from that be numbered 1, 2, 3, &c. In like manner, let the same divisions be continued above the boiling point, as far as the tube will admit.

It is evident that, under these circumstances, the freezing point will be marked by 32, and the boiling point by 212. It is usual to express the degrees of a thermometer in the same manner as the degrees of a circle, by placing a small ° above the number. Thus the freezing point is expressed by 32°, and the boiling point by 212°.

The reason the degrees were commenced at 32° below the freezing point was, because, when the thermometer was invented, that temperature was supposed to be the lowest degree of cold possible, being that of a certain mixture of [Pg100] snow and salt. This, however, has since been found to be an error, very much lower temperatures being obtained by various physical expedients.

The temperature of a body is, then, that elevation to which the thermometer would rise when immersed in that body. Thus, if in plunging the thermometer in water we found the mercury to rise or fall to the division marked 100, we should then say, the temperature of the water was 100°.

Let us suppose a spirit lamp, or other regular source of heat, applied to a bath of mercury, so as to maintain the mercury at a fixed temperature of 200°, and let another vessel, containing a quantity of ice at a temperature of 20° be immersed in the mercury. Let a thermometer be placed in the mercury, and another in the ice. The following effects will then ensue. The thermometer immersed in the ice will be observed gradually to rise from 20° upwards, until it indicates the temperature of 32°. It will then become stationary, and the ice which had hitherto remained in a solid state will begin to melt and be converted into water. This process of liquefaction will continue for a considerable time, during which the thermometer immersed in the ice will constantly be maintained at 32°. At the moment, however, when the last portion of ice is liquefied, the thermometer will begin again to rise. The coincidence of this ascent of the thermometer with the completion of the liquefaction of the ice, may be very easily observed, because the ice being lighter, bulk for bulk, than water, will float on the surface, and so long as a particle of it remains unmelted it will be distinctly seen.

Now it cannot be doubted that, during the whole of this process, the mercury, supposed to be maintained at 200°, constantly imparts heat to the ice; yet, from the moment the liquefaction begins, until it is completed, no increased temperature is exhibited by the thermometer immersed in the melting ice. If during this part of the process no heat were received by the ice from the mercury, the consequence would be, that the application of the lamp would cause the temperature of the mercury to rise above 200°, which may be easily demonstrated by withdrawing the vessel of ice from the mercurial bath during the process of liquefaction. The moment [Pg101] 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 [Pg102] 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 twenty-eight 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.