MRS. B.
Yes; but though feathers in general are an excellent preservative against cold, down is a kind of plumage peculiar to aquatic birds, and covers their chest, which is the part most exposed to the water; for though the surface of the water is not of a lower temperature than the atmosphere, yet, as it is a better conductor of heat, it feels much colder, consequently the chest of the bird requires a warmer covering than any other part of its body. Besides, the b.r.e.a.s.t.s of aquatic birds are exposed to cold not only from the temperature of the water, but also from the velocity with which the breast of the bird strikes against it; and likewise from the rapid evaporation occasioned in that part by the air against which it strikes, after it has been moistened by dipping from time to time into the water.
If you hold a finger of one hand motionless in a gla.s.s of water, and at the same time move a finger of the other hand swiftly through water of the same temperature, a different sensation will be soon perceived in the different fingers.
Most animal substances, especially those which Providence has a.s.signed as a covering for animals, such as fur, wool, hair, skin, &c. are bad conductors of heat, and are, on that account, such excellent preservatives against the inclemency of winter, that our warmest apparel is made of these materials.
EMILY.
Wood is, I dare say, not so good a conductor as metal, and it is for that reason, no doubt, that silver teapots have always wooden handles.
MRS. B.
Yes; and it is the facility with which metals conduct caloric that made you suppose that a silver pot radiated more caloric than an earthen one.
The silver pot is in fact hotter to the hand when in contact with it; but it is because its conducting power more than counterbalances its deficiency in regard to radiation.
We have observed that the most dense bodies are in general the best conductors; and metals, you know, are of that cla.s.s. Porous bodies, such as the earths and wood, are worse conductors, chiefly, I believe, on account of their pores being filled with air; for air is a remarkably bad conductor.
CAROLINE.
It is a very fortunate circ.u.mstance that air should be a bad conductor, as it tends to preserve the heat of the body when exposed to cold weather.
MRS. B.
It is one of the many benevolent dispensations of Providence, in order to soften the inclemency of the seasons, and to render almost all climates habitable to man.
In fluids of different densities, the power of conducting heat varies no less remarkably; if you dip your hand into this vessel full of mercury, you will scarcely conceive that its temperature is not lower than that of the atmosphere.
CAROLINE.
Indeed I know not how to believe it, it feels so extremely cold. --But we may easily ascertain its true temperature by the thermometer. --It is really not colder than the air;--the apparent difference then is produced merely by the difference of the conducting power in mercury and in air.
MRS. B.
Yes; hence you may judge how little the sense of feeling is to be relied on as a test of the temperature of bodies, and how necessary a thermometer is for that purpose.
It has indeed been doubted whether fluids have the power of conducting caloric in the same manner as solid bodies. Count Rumford, a very few years since, attempted to prove, by a variety of experiments, that fluids, when at rest, were not at all endowed with this property.
CAROLINE.
How is that possible, since they are capable of imparting cold or heat to us; for if they did not conduct heat, they would neither take it from, nor give it to us?
MRS. B.
Count Rumford did not mean to say that fluids would not communicate their heat to solid bodies; but only that heat does not pervade fluids, that is to say, is not transmitted from one particle of a fluid to another, in the same manner as in solid bodies.
EMILY.
But when you heat a vessel of water over the fire, if the particles of water do not communicate heat to each other, how does the water become hot throughout?
MRS. B.
By constant agitation. Water, as you have seen, expands by heat in the same manner as solid bodies; the heated particles of water, therefore, at the bottom of the vessel, become specifically lighter than the rest of the liquid, and consequently ascend to the surface, where, parting with some of their heat to the colder atmosphere, they are condensed, and give way to a fresh succession of heated particles ascending from the bottom, which having thrown off their heat at the surface, are in their turn displaced. Thus every particle is successively heated at the bottom, and cooled at the surface of the liquid; but as the fire communicates heat more rapidly than the atmosphere cools the succession of surfaces, the whole of the liquid in time becomes heated.
CAROLINE.
This accounts most ingeniously for the propagation of heat upwards. But suppose you were to heat the upper surface of a liquid, the particles being specifically lighter than those below, could not descend: how therefore would the heat be communicated downwards?
MRS. B.
If there were no agitation to force the heated surface downwards, Count Rumford a.s.sures us that the heat would not descend. In proof of this he succeeded in making the upper surface of a vessel of water boil and evaporate, while a cake of ice remained frozen at the bottom.
CAROLINE.
That is very extraordinary indeed!
MRS. B.
It appears so, because we are not accustomed to heat liquids by their upper surface; but you will understand this theory better if I show you the internal motion that takes place in liquids when they experience a change of temperature. The motion of the liquid itself is indeed invisible from the extreme minuteness of its particles; but if you mix with it any coloured dust, or powder, of nearly the same specific gravity as the liquid, you may judge of the internal motion of the latter by that of the coloured dust it contains. --Do you see the small pieces of amber moving about in the liquid contained in this phial?
CAROLINE.
Yes, perfectly.
MRS. B.
We shall now immerse the phial in a gla.s.s of hot water, and the motion of the liquid will be shown, by that which it communicates to the amber.
EMILY.
I see two currents, the one rising along the sides of the phial, the other descending in the centre: but I do not understand the reason of this.
MRS. B.
The hot water communicates its caloric, through the medium of the phial, to the particles of the fluid nearest to the gla.s.s; these dilate and ascend laterally to the surface, where, in parting with their heat, they are condensed, and in descending, form the central current.
CAROLINE.
This is indeed a very clear and satisfactory experiment; but how much slower the currents now move than they did at first?
MRS. B.
It is because the circulation of particles has nearly produced an equilibrium of temperature between the liquid in the gla.s.s and that in the phial.
CAROLINE.
But these communicate laterally, and I thought that heat in liquids could be propagated only upwards.
MRS. B.