Philosophical transactions, Vol. L. Part I. For the year 1757. · Various
+LETTER I.+
_To the very honourable and learned the_ PRESIDENT _and_ MEMBERS _of the_ Royal Society. [Read May 5, 1757.] Gentlemen, THERE is ever such a charm attendant upon novelty, that be it in philosophy, medicine, or religion, the gazing world are too often led to adore, what they ought only to admire: whilst this vehemence of enthusiasm has generally soon rendered that object contemptible, that would otherwise have long laid claim to a more sober esteem. This was once the fate of chemistry: the vain and pompous boasts of her adepts brought the whole art into disrespect; and I should be sorry, if her sister electricity should share the same misfortunes. It is hence the ingenious Mr. Eeles will excuse me, for endeavouring to lay before you my opinion on the ascent of vapours, tho’ it by no means coincides with that he is so strenuous to establish, and plucks a plume from his idol goddess electricity. The probability, supporting the hypothesis of Mr. Eeles, according to his own expressions, rests on this: “That every particle of vapour is endued with a portion of electric fire; and that there is no other sufficient cause assigned for their ascending.” (_Phil. Trans. vol._ xlix. _part._ i. _p._ 134.). My design is therefore first to attempt to shew, that another theory, founded on principles better known, will sufficiently explain the ascent of vapours: and then, that some kinds of vapours are not endued with a more or less than their natural share of electric æther.
The immense rarefaction of explosive bodies by heat, depends either on the escape of air before condensed in them, or on the expansion of the constituent parts of those bodies. This distinction has not been sufficiently considered by any one to my knowlege; nor shall I at present amuse the Society upon this head; it being enough for my present purpose to observe, that they may be thus distinguished: where air is emitted, it cannot be condensed again into the same bulk by cold; but the expansion of heated parts of bodies, as soon as that heat is withdrawn, ceases to exist. Nitre comes under the first of these classes: in detonation it emits great quantities of air, not afterwards condensible to the like space. This may be seen by firing a few grains of gunpowder in an unblown bladder, or in a vessel nearly full of water with its mouth inverted. The same is true of all the solid parts of animals and vegetables, when subjected to fire; as appears from the experiments of that learned philanthropist, Dr. Hales. But of water the contrary is evident. In the steam-engine, a jet of cold water, we find, instantly condenses that immense rarefaction; which I apprehend could not be, if it was constituted of escaped elastic air. And altho’ this steam must be acknowledged to put on some properties of air; such as ventilating a fire; or that a taper blown out by it, is capable of being again lighted immediately, and that without a crackling noise, which occurs when touched with water; this does not in the least invalidate our opinion, tho’ it has certainly conduced very much to propagate the former one: since from this way of reasoning, the whole must be air, and we should have no water at all in vapour.
From considering this power of expansion, which the constituent parts of some bodies acquire by heat; and withal, that some bodies have a greater affinity to heat, that is, acquire it sooner and retain it longer than others; which affinity appears from experiments, and which, I apprehend, is in some ratio of their specific gravities and their powers of refraction, reflexion, or absorption of light; or at least in some ratio much greater than that of their specific gravities alone. From considering these, I say, many things, before utterly inexplicable, became easily understood by me. Such as, Why when bismuth and zinc are fused together, and set to cool, the zinc, which is specifically heavier, is found above the bismuth? Why the buff covering of inflammatory blood, the skum of heated milk, the sedative salt of borax, which are all specifically heavier than the liquids in which they are formed, are still formed at the surface of them? How benzoin, sulphur, and even the ponderous body mercury, may be raised into vapour, again to be condensed unaltered? And, lastly, how water, whose parts appear from the æolipile to be capable of immeasurable expansion, should by heat alone become specifically lighter than the common atmosphere, without having recourse to a shell inclosing air, or other assistant machinery? and when raised, I am persuaded we shall find, that to support them floating, perhaps many days, in the atmosphere, is not a knot so intricate, as to oblige us to conjure up a new divinity to unravel it.
But before we proceed to this second part of our task, it will be necessary previously to consider, first, how small a degree of heat is required to detach or raise the vapour of water from its parent-fluid. In the coldest day, I might say the coldest night, of winter, when the weather is not frosty or very damp, wet linen or paper will become dry in the course of a few hours. A greater degree of heat must indeed cause a quicker evaporation. But I am persuaded, that was it not for the pressure of the superincumbent fluid, greatly less than that of boiling water would instantly disperse the whole so heated into vapour. Secondly, That in the opinion of Sir Isaac Newton, well illustrated by the late lamented Mr. Melvil, the sun-beams appear only to communicate heat to bodies by which they are refracted, reflected, or obstructed; whence, by their impulse, a reaction or vibration is caused in the parts of such impacted bodies.
This is supported by the experiment of approaching some light body, or blowing smoke near the focus of the largest glasses; and from observing, that these do not ascend, it is evident the air is not so much as warmed by the passage of those beams thro’ it, yet would instantly calcine or vitrify every opake body in nature. And from this we may collect, that transparent bodies are only heated at their surfaces, and that perhaps in proportion to their quantity of refraction: which will further give and receive illustration from those very curious experiments, of producing cold by the evaporation of liquors, published by the learned Dr. Cullen, in the late volume of Essays Physical and Literary, at Edinburgh. In these experiments a spirit-thermometer was immersed in spirit of wine, and being suddenly retracted, was again exposed to the air; and as the spirit of wine adhering to the glass evaporated, the spirit contained within the thermometer was observed to subside. Now as the difference of the refraction of spirit of wine and glass is exceedingly minute, compared with the difference of refraction of spirit of wine and air; we may consider, in the above experiment, the heat to be communicated to the thermometer only at its surface: but here the adherent fluid escapes as soon as heated; by which means the glass, and its contents, are deprived of that constant addition of heat, which other bodies perpetually enjoy either from the sun-beams immediately, or from the emanations of other contiguous warmer bodies; and must thence, in a few minutes, become colder than before.
The ingenious Mr. Eeles, I dare say, has already foreseen the use I am going to make of this principle; _viz._ “That the little spherules of vapour will thus, by refracting the solar rays, acquire a constant heat, tho’ the surrounding atmosphere remain cold.” And as from the minuteness of their diameters, if they are allowed to be globules, they must do this to a very great degree, I apprehend none of those objections will take place against us, with which Mr. Eeles has so sensibly confuted the former received theories on this subject. If we are asked, how clouds come to be supported in the absence of the sun? It must be remembered, that large masses of vapour must for a considerable time retain much of the heat they have acquired in the day; at the same time reflecting, how small a quantity of heat was necessary to raise them; and that doubtless even a less will be sufficient to support them, as from the diminished pressure of the atmosphere at a given height, a less power may be able to continue them in their present state of rarefaction; and, lastly, that clouds of particular shapes will be sustained or elevated by the motion they acquire from winds.
I should here have concluded this paper, perhaps already too long; but upon revising it, I find, where the affinity of some bodies with heat is mentioned, that the deductions made from thence are not sufficiently explained to be intelligible. First then, If the power of expansion of any two bodies, by heat, be in a greater proportion than their specific gravities, then will there be a certain degree of heat, in which their specific gravities will be equal; and another, in which the gravity of that, which was lighter when cold, will exceed the gravity of that, which was heavier when cold. Hence zinc and bismuth alter their specific gravities in fusion; some urine, and many solutions of solids, grow turbid as they cool; others alter their colours. Secondly, If (the power of expansion by heat being equal) the power of retaining heat be in a greater ratio than the specific gravities; then, during the time of cooling after being sufficiently heated, there will be an instant, when the heavier body will become the lighter, and swim upon the other. This seems the case in the buff covering of inflamed blood, the skum of heated milk, and the cristallization of some salts: for if these effects were from the evaporation of the thinner parts at the surface, they should happen during the greatest evaporation, or when boiling; but, on the contrary, they are all done in the greatest degree when the liquor has for some time began to cool. Lastly, If the quickness of acquiring heat be in a greater proportion than their specific gravities (the power of expansion being equal), then, during the time of their acquiring heat, there will be an instant, when the body, that was heavier when cold, will now become the lighter. From one or more of which principles, I apprehend, the volatility or fixity of all minerals, and many other bodies, takes its origin.
It is no part of my design to account to you, gentlemen, in what manner such an expansion of the parts of bodies can be brought about by the action of fire. Tho’ perhaps a rotatory motion only of each particle on its own center might be sufficient to produce such a rarefaction; and the more so, if such parts were any other figures than spheres, as by the percussion of their angles they must result further from each other. Nor is the existence of such a rotatory motion without some probability, when we observe the verticillary motion given to charcoal-dust thrown on nitre in fusion, or the wonderful agitation of the parts of burning phosphorus, or even of a common red letter-wafer touched by the flame of a candle. But as in this paper I have laboured (and I hope not without success) to shew you, that some properties of solar heat are sufficient to account for the elevation and support of vapours; so in another letter I propose nearly to demonstrate to you, that the electric æther is far from having any share in the production of this important phænomenon. From, GENTLEMEN, Your very humble Servant, Erasmus Darwin. Litchfield, Mar. 20, 1757.
Ideas discussed in this chapter
Text from Project Gutenberg, public domain in the United States.