The Steam Engine Familiarly Explained and Illustrated. Dionysius Lardner
the zero, and are expressed, like the degrees of a circle, by placing a small ° over the number. Thus it will be perceived that the freezing point is 32° of our thermometer, and the boiling-point will be found by adding 180° to 32°; it is therefore 212°.
The temperature of a body is that elevation to which the thermometer would rise when the mercury enclosed in it would acquire the same temperature. Thus, if we should immerse the thermometer, and should find that the mercury would rise to the division marked 100°, we should then affirm that the temperature of the water was 100°.
(9.) The dilatation which attends an increase of temperature is one of the most universal effects of heat. It varies, however, in different bodies: it is least in solid bodies; greater in liquids; and greatest of all in bodies in the aeriform state. Again, different solids are differently susceptible of this expansion. Metals are the most susceptible of it; but metals of different kinds are differently expansible.
As an increase of temperature causes an increase of bulk, so a diminution of temperature causes a corresponding diminution of bulk, and the same body always has the same bulk at the same temperature.
A flaccid bladder, containing a small quantity of air, will, when heated, become quite distended; but it will again resume its flaccid appearance when cold. A corked bottle of fermented liquor, placed before the fire, will burst by the effort of the air contained in it to expand when heated.
Let the tube A B (fig. 5.) open at both ends, have one end inserted in the neck of a vessel C D, containing a coloured liquid, with common air above it; and let the tube be fixed so as to be air-tight in the neck: upon heating the vessel, the warm air inclosed in the vessel C D above the liquid will begin to expand, and will press upon the surface of the liquid, so as to force it up in the tube A B.
In bridges and other structures, formed of iron, mechanical provisions are introduced to prevent the fracture or strain which would take place by the expansion and contraction which the metal must undergo by the changes of temperature at different seasons of the year, and even at different hours of the day.
Thus all nature, animate and inanimate, organized and unorganized, may be considered to be incessantly breathing heat; at one moment drawing in that principle through all its dimensions, and at another moment dismissing it.
(10.) Change of bulk, however, is not the only nor the most striking effect which attends the increase or diminution of the quantity of heat in a body. In some cases, a total change of form and of mechanical qualities is effected by it. If heat be imparted in sufficient quantity to a solid body, that body, after a certain time, will be converted into a liquid. And again, if heat be imparted in sufficient quantity to this liquid, it will cease to exist in the liquid state, and pass into the form of vapour.
By the abstraction of heat, a series of changes will be produced in the opposite order. If from the vapour produced in this case, a sufficient quantity of heat be taken, it will return to the liquid state; and if again from this liquid heat be further abstracted, it will at length resume its original solid state.
The transmission of a body from the solid to the liquid state, by the application of heat, is called fusion or liquefaction, and the body is said to be fused, liquefied, or melted.
The reciprocal transmission from the liquid to the solid state, is called congelation, or solidification; and the liquid is said to be congealed or solidified.
The transmission of a body from the liquid to the vaporous or aeriform state, is called vaporization, and the liquid is said to be vaporized or evaporated.
The reciprocal transmission of vapour to the liquid state is called condensation, and the vapour is said to be condensed.
We shall now examine more minutely the circumstances which attend these remarkable and important changes in the state of body.
(11.) Let us suppose that a thermometer is imbedded in any solid body; for example, in a mass of sulphur; and that it stands at the ordinary temperature of 60 degrees: let the sulphur be placed in a vessel, and exposed to the action of fire. The thermometer will now be observed gradually to rise, and it will continue to rise until it exhibit the temperature of 218°. Here, however, notwithstanding the continued action of the fire upon the sulphur, the thermometer will become stationary; proving, that notwithstanding the supply of heat received from the fire, the sulphur has ceased to become hotter. At the moment that the thermometer attains this stationary point, it will be observed that the sulphur has commenced the process of fusion; and this process will be continued, the thermometer being stationary, until the whole mass has been liquefied. The moment the liquefaction is complete, the thermometer will be observed again to rise, and it will continue to rise until it attain the elevation of 570°. Here, however, it will once more become stationary; and notwithstanding the heat supplied to the sulphur by the fire, the liquid will cease to become hotter: when this happens, the sulphur will boil; and if it continue to be exposed to the fire a sufficient length of time, it will be found that its quantity will gradually diminish, until at length it will all disappear from the vessel which contained it. The sulphur will, in fact, be converted into vapour.
From this process we infer, that all the heat supplied during the processes of liquefaction and vaporization is consumed in effecting these changes in the state of the body; and that under such circumstances, it does not increase the temperature of the body on which the change is produced.
These effects are general: all solid bodies would pass into the liquid state by a sufficient application of heat; and all liquid bodies would pass into the vaporous state by the same means. In all cases the thermometer would be stationary during these changes, and consequently the temperature of the body, in those periods, would be maintained unaltered.
(12.) Solids differ from one another in the temperatures at which they become liquid. These temperatures are called their melting points. Thus the melting point of ice is 32°; that of lead 612°; that of gold 5237°.[3] The heat which is supplied to a body during the processes of fusion or vaporization, and which does not affect the thermometer, or increase the temperature of the body fused or vaporized, is said to become latent. It can be proved to exist in the body fused or vaporized, and may even be taken from that body. In parting with it the body does not fall in temperature, and consequently the loss of this heat is not indicated by the thermometer any more than its reception. The term latent heat is merely intended to express this fact, of the thermometer being insensible to the presence or absence of this portion of heat, and is not intended to express any theoretical notions concerning it.
(13.) In explaining the construction and operation of the steam engine, although it is necessary occasionally to refer to the effects of heat upon bodies in general, yet the body, which is by far the most important to be attended to, so far as the effects of heat upon it are concerned, is water. This body is observed to exist in the three different states, the solid, the liquid, and the vaporous, according to the varying temperature to which it is exposed. All the circumstances which have been explained in reference to metals, and the substance sulphur in particular, will, mutatis mutandis, be applicable to water. But in order perfectly to comprehend the properties of the steam engine, it is necessary to render a more rigorous and exact account of these phenomena, so far as they apply to the changes produced upon water by the effects of heat.
Let us suppose a mass of ice immersed in the mixture of snow and salt which determines the zero point of the thermometer: this mass, if allowed to continue a sufficient length of time submerged in the mixture, will necessarily acquire its temperature, and the thermometer immersed in it will stand at zero. Let the ice be now withdrawn from the mixture, still keeping the thermometer immersed in it, and let it be exposed to the atmosphere at the ordinary temperature, say 60°. At first the thermometer will be observed gradually and continuously to rise until it attain the elevation of 32°; it will then become stationary, and the ice will begin to melt: the thermometer will continue standing at 32° until the ice shall be completely liquefied. The liquid ice and the thermometer being contained in the same vessel, it will be found, when the liquefaction is completed, that the thermometer will again begin to rise, and will continue