Imágenes de página
PDF
ePub

solid bodies, though the proofs are not so common; since, as I have just observed, the particles of solid bodies have less power of movement, and, consequently, of adaptation to each other, than those of liquids. Thus, two plates of lead, whose opposite surfaces correspond so exactly that every particle of each surface shall have a bearing upon the particle opposed to it, when once united by pressure, assisted by a little friction, cohere so powerfully as to require a very considerable force to separate them. And it may be shown, either by measuring this force, or by suspending the lead in the vacuum of an air-pump, that the pressure of the atmosphere is not materially concerned in producing this effect. A cohesion of this kind is sometimes of practical utility in the arts; little ornaments of laminated silver remaining attached to iron or steel, with which they have been made to connect themselves by the powerful pressure of a blow, so as to form one mass with it. And it is now a well-known fact, and of a most curious nature, that one of the causes by which eight-day clocks go at times irregularly, and monthly clocks, whose weights are much larger and heavier, often amounting to not less than thirty pounds, stop suddenly, proceeds from the attraction which takes place between their leaden weights and the leaden ball of the pendulum, when the weights have descended just so low as to be on a level, and, consequently, very nearly in a state of contact, with the pendulum-ball. And hence the reason why both these kinds of clocks, if the pendulum have not actually stopped, seem gradually, a few days afterward, to recover their former accuracy; the attraction diminishing as the distance once more increases.* In like manner, Studor remarks that beams of steel become sometimes erroneous by acquiring magnetic polarity.j

It is by the same means that the greater number of rocks seem to be produced that enter into the substance of the earth's solid crust. The lowermost of these, as I shall have occasion to observe in an ensuing lecture, are united by an intimate crystallization, which is the most perfect form of aggregate or homogeneous attraction that can exist between solid bodies, and which must have commenced while such bodies were in a fluid state. Some of the upper kinds or families are united by a particular cement, which is nothing more than a substance possessing a peculiar attraction, or, if I may be allowed the expression, physical partiality to the rudimental corpuscles of which the rock consists; and others by nothing more than the law of aggregation or homogeneous attraction in its simplest state; whence earths unite to earths in consequence of mutual approximation, assisted by theit own or a superincumbent pressure, in the same manner as 1 have just stated that plates of lead or other metals unite to metals. II. But there are substances that are Unlike In Their Nature, as solids and fluids, for instance, that under particular circumstances are often found to exhibit a mutual attraction; whence this mode of union is called HeterogeNeous Attraction, and from its occurring most palpably between liquids and solid substances possessing small capillary or hair-tubes, Capillary AtTraction.

The cause of this attraction is obvious; and it is still more clearly a mere modification of the general attraction of gravitation, than the preceding power of homogeneous attraction. It is the common attractive property of material substance for material substance; the liquid, or that whose particles are easily separable, pressing toward the solid, whose parts'are by any action of their own altogether inseparable. Hence the reason why water or any other liquid hangs about the sides of a wine-glass: hence, partly, the reason why a wineglass, when somewhat more than brim-full of a liquid, does not overflow ; the co-operative reason being, as I have already stated, the homogeneous attraction of the corpuscles of the fluid for each other, which prevents them from separating readily: and hence also the reason why a liquid contained in a narrownecked and inverted phial does not obey the common attraction of gravitation, and fall to the earth, although the stopper be removed to allow it, till we

[ocr errors][merged small][ocr errors]

aid the power of gravitation, or rather loosen the power of the peculiar attraction, by shaking the phial. In this last case it is manifest that the heterogeneous attraction, or that between the two different substances, is stronger than the common force of gravity. In minute capillary tubes or pores this is still more obvious. Such are the pores of a piece of sponge, when pressed or softened, so as to become more pliable to the action of water or of any other liquid within its reach. For, in this case, the water being minutely divided by the pores of the sponge into very small portions, and still surrounded by the pores in every direction after such division, has its common force of gravitation and its peculiar force of homogeneous attraction equally overpowered; and ascends from the surface of the earth, instead of descending to it, or uniting into a spherical form; and the same kind of pores, and, consequently, the same kind of power, being continued to the utmost height of the sponge, it will rise to the full extent of its column. The tubes of various imperfect crystals, as those of sugar, for example, are still smaller; and hence the lateral attraction must be still stronger; and any liquid within its reach will rise both higher and more freely, till the sugar at length becomes dissolved, and, consequently, its pores are totally destroyed. The cause of capillary attraction is therefore obvious: and the reasoning and phenomena now submitted may be applied to an explanation of every other species of the same kind that may occur to us. III. The third particular attraction 1 have noticed, is that of Peculiar Bodies For Peculiar Bodies, and which has hence been denominated Elective or Chemical Attraction; as the tendencies they have to each other have been denominated Affinities. Thus limehas a strong affinity for carbonic acid, and greedily attracts it from the atmosphere, which hence becomes purified by being deprived of it. But the same substance has a still stronger affinity for sulphuric acid, and hence parts with its carbonic acid, which flies off in the form of gas, in order to unite with the sulphuric whenever it has a possibility of doing so. It is highly probable that this kind of attraction is also nothing more than a peculiar modification of that of gravitation, more select in its range, but more active in its power. To trace out the various substances that are possessed of this peculiar property, and to measure the degrees of their affinities, is one of the chief branches of chemistry, but of too voluminous a nature to touch farther upon at present. IV. V. The two remaining kinds of attraction to which I have adverted, those of Electricity and of Magnetism, are still more select, and perhaps still more powerful than even the preceding: but the phenomena to which they give rise cannot, I think, be attributed to any modification of a gravitating ethereal medium. We call the medium in both these cases a fluid, but we know little or nothing of the laws by which they are regulated; whether they be different substances, or, according to M. Ampore, the same substance under different modifications, or whether, in reality, they be material substances at all. They are certainly deficient in the most obvious properties of common matter, and may be another substrate of being united to it. There are also two other substances, or which are generally conceived to be substances, in nature, of a very attenuate texture, which largely contribute to the changes of material bodies. I mean Light and Heat, of the general nature of which we are still also in a considerable degree of ignorance. Like the powers of magnetism and electricity, we only know them, and can only reason concerning them, by their effects. These effects, indeed, are of a most curious and interesting character, but spread too widely to be followed up in the course of the present lecture, though we may endeavour to pursue them, and, as far as we are able, to develope them, hereafter. All these four powers or essences, for we know not which to call them, concur in exhibiting none of the common properties of matter; their respective particles repel each other at least as powerfully as they attract, and in the cases of light and heat repel alone, and without attracting. They may, possibly, be ponderable; but if so, we have no instruments fine enough to detect their relative weights; and we are hence incapable of determining, as I took leave to observe on a former occasion, whether they be matter at all, whether mere properties of matter, or whether modifications of some ethercalised and incorporeal substrate, combining itself with the material mass, and exciting many of its most extraordinary phenomena. It is at present, however, very much the habit to generalise them into one common origin; and to conceive the whole as modified results of matter, or of the gravitating property of matter. Thus, the attractive powers of chemical affinity and of electricity are identified in the following passage of Sir Humphry Davy's valuable " Elements of Chemical Philosophy:"—" Electrical effects are exhibited by the same bodies when acting as masses, which produce chemical phenomena when acting by their particles; it is not improbable, therefore, that the primary cause of both may be the same."* And in like manner, in an adjoining passage, he suggests that all the various properties or essences that have thus far passed in survey before us, may be nothing more than the general attractive power of matter, though he admits that at present we are incompetent to determine upon the subject. "With regard to the great speculative questions, whether the electrical phenomena depend upon one fluid in excess in the bodies positively electrified, and in deficiency in the bodies negatively electrified, or upon two different fluids capable by their combination of producing heat and light, or whether they may be particular exertions of the general attractive power of matter, it is, perhaps, impossible to decide, in the present imperfect state of our knowledge."f And hence, heat, in the view of Sir Humphry Davy, Count Rumford, and various other justly celebrated chemists and philosophers of the present day, coincidently with the doctrine of the Peripatetic school, is a mere property of matter, and not a substance sui generis, as was contended for by the Epicureans, in opposition to the disciples of Aristotle, and is contended for by the disciples of Boerhaave, Black, Crawford, and most of the chemists of our own times. The cause of heat, among those who deny it a substantive existence, consists in a vibrating motion of the constituent particles of the heated body, too rapid to be traced by the eye. And as it is known to every one that bodies in general, as they become heated, occupy a larger space, and have their particles more widely repelled and separated from each other than in a colder temperature, it has of late become a favourite doctrine that the repulsive power, which in our last lecture we noticed to exist throughout matter, depends altogether upon the property of heat; in consequence of which Sir Humphry Davy uses heat and calorific repulsion as synonymous terms, and hence regards heat and gravitation, or general attraction, as antagonist powers. There is much plausible reasoning to be urged in favour of this hypothesis. It will as readily account for many, perhaps most, of the phenomena which accompany bodies in their change from one temperature to another, as the position of the substantive form of heat, and has some advantage in point of simplicity; but it is opposed by a variety of facts of so stubborn and intractable a nature, that no efforts of ingenuity have hitherto been capable of bending them into the service of the new doctrine. I observed, for instance, in our last lecture, that when two plates of glass are within a ten thousandth part of an inch of each other, they cannot be made to approach nearer without a strong additional pressure. I observed, farther, that Professor Robison has calculated the extent of this pressure from actual experiment, and finds it amount to not less than a thousand pounds weight for every square inch of the glass. Now this resistance or repulsive power between the two plates of glass takes place equally under an air-pump and in the fullest exposure to the air of the atmosphere, but it appears to cease under water. By what cause the repulsion is excited in the two former instances, or disappears in the latter, we know not; but it does not seem possible for any ingenuity of argument to connect this repulsive power with heat, whether regarded as a substance or a mere property.

*Eim-p-Im,i85 trap. 178

Heat, again, which undoubtedly makes the particles of iron repel each other, so that given weights of them occupy a larger space—makes the particles of a ball of clay, on the contrary, attract each other into a closer approximation, so as very considerably to lessen its dimensions; and it was on account of this peculiar property that Mr. Wedgewood selected this last material for the purpose of forming his celebrated pyrometer, or instrument for measuring intense heats, the increase of the heat being indicated by the decrease of the mass of clay.

'So water at about 42° of Fahrenheit, which forms its medium of density, begins to expand upon exposure to heat, and continues to expand in proportion as additional heat is applied; but below 42° it begins to expand also upon exposure to cold, and continues to expand in the very same ratio upon the application of additional cold, till at 32° it freezes and becomes fixed. This curious phenomenon has never been accounted for. If calorific repulsion produce the expansion above 42°, what is it that produces the same effect below? We can, perhaps, explain the cause of the expansion during the act of freezing, from the peculiar shape of the crystals which the water assumes in the act of consolidating; but this explanation will in no respect apply to the expansion of the water when it reaches the freezing point. In this curious and unillustrated fact cold appears to be as much entitled to the character of a repulsive power as heat.- j

For these and numerous other reasons, therefore, heat is even at the present moment usually regarded, not as a mere quality of body produced by internal vibration, and forming an antagonist power to the attraction of cohesion, but as a distinct and independent substance. The sources of heat are various, though by far the principal reservoir throughout the whole solar system is the sun himself, which Dr. Herschel believes to be perpetually secreting the matter of heat from those dark and discoloured parts on its surface which we call spots, by many astronomers regarded as volcanoes, and many of which are larger than, and some of them five or six times as large as the diameter of the earth! This material Dr. Herschel supposes to be first thrown off in the form of an atmosphere, and afterward this atmosphere to be diffused in every direction through the whole range of the solar empire: and, in the Philosophical Transactions for 1801, he has endeavoured to show that the variation in the heat of different years is owing to the more or less copious supply of fuel which such spots communicate. This opinion I at present merely glance at; as it is my intention on a future occasion to examine its validity, as well as to trace out the other sources from which heat is derived, and to take a survey of the laws by which it is regulated. It will form a progressive part of that investigation to follow up the general nature of light; to try the question whether it be a substance or a property; and if a substance, whether distinct from or a mere modification of heat. I shall at present only observe, that, in one of the latest opinions of the philosopher to whom I have just adverted, it is not only a substance, but the source of all visible substances, and the basis of all worlds. Dr. Herschel has recently taken great pains to prove, but with no small degree of repugnancy to a former hypothesis of his, that the luminous fluid which so often appears in the heavens on a bright night, and shoots streaks athwart them, is diffused light, existing independently of suns or stars, though perhaps originally thrown forth from them; another kind of ethereal matter being sometimes united with that of light, and hence rendering it at times capable of opacity. In this diffused state he calls every distinct mass a nebulosity; he conceives all its particles to be subject to the common laws of gravitation, or the centripetal force; and that certain circumstances, unknown to us, may have occasionally produced a nearer approximation between some particles than between others; whence the diffused nebulosity is, in such part, converted into a denser nucleus, which by its comparative prcponderancy, must lay a foundation for a rotatory motion, and attract and determine the circumjacent matter still more closely to itself, and consequently, diminish the extent of the nebulous range. The nuclei thus arising may sometimes be double or triple, or still more complicated; and whenever this occurs, the nebulosity will be broken into different nebulae, or smaller nebulous clouds; and if some of them be much minuter than others, the minuter may at length attend upon the larger, as satellites upon a planet: and Dr. Herschel gives instances of all these phenomena actually completed, or in a train of completion, in different parts of the visible heavens. Such he submits as his latest opinion of the general construction of the heavens; believing stars, planets, and comets to have originated, and to be still originating, from such a source; the nebulous matter contained in a cubical space seen under an angle of ten degrees demanding a condensation of two trillion and two hundred and eight thousand billion times before it can be so concentrated as to constitute a globe of the diameter and density of our sun. Some of these masses of light are indistinct and barely visible even by Dr. Herschel's forty feet telescope; and he hence calculates, that if a mass thus traced out contain a cluster of five thousand stars, they must be eleven millions of millions of millions of miles off. M. Huygens entertained an analogous idea: and conceived that there are stars so immensely remote, that their light, although travelling at the rate of eleven millions of miles in a minute, and having thus continued to travel from the formation of the earth, or for nearly six thousand years, has not yet reached us. But this sublime conception is of much earlier origin; and it is due to the magnificence of the Epicurean scheme to state that it is to be found completely developed among its principles. Lucretius has beautifully alluded to it in lines of which I must beg your acceptance of the following feeble translation, the only difference being, that lightning or the electric fluid, is here employed instead of light, at least by Havercamp; for Vossius, in the Leyden edition, gives us light for lightning, reading lumina instead offidmina.

The poet is speaking of the immensity of space:— The vast whole What fancied scene can bound? O'er its broad realm, Inimeasur'd, and immeasurably spread, From age to age resplendent lightnings urge, In vain, their night perpetual; distant, still, And ever distant from the verge of things, So vast the space or opening space that swells, Through every part so infinite alike.* From this immense range of nebulous light Dr. Herschel derives comets, as well as stars and planets, believing them, indeed, to be the rudiments of the two latter; and he has especially noticed, as originating from this source, the well-remembered comet that so brilliantly, and forso long a period of time, visited our horizon during the close of the year 1811; which he conceives will be converted into a stellar or planetary orb as soon as its luminous matter, and especially that of its enormous tail, shall be sufficiently concentrated for this purpose. This tail he calculated, when at its greatest apparent stretch in October of the same year, at something more than a hundred millions of miles long, and nearly fifteen millions broad, though its bright or solid nucleus or planetary body was not supposed to measure more than four hundred and twenty-eight miles. Its perihelion path, or nearest approach to the sun, is stated at a distance of ninety-seven millions of miles, its distance from the earth at ninety-three millions. The comet of 1807 approached the earth within sixty-one millions of miles, or about a third nearer the earth, and that * Omne quidem vcro nihil est quod flnlat extra. Est igitur nature lor.i, spatiumque profundi. Quod neque clara suo percurrere fulmlna cursu Perpetun possint nvi labentia tractu;

Nec proraum facere, ut restet minus Ire, meando

Usque adeo passim patet ingens copia rebus,

FUiibus exemptis, in cunctas undiquc paneis

't D» *tor. Nat. i. 1000.

« AnteriorContinuar »