material superficies, and yet, according to the Farradian definition, reaches beyond the moon!

1821. But the idea of that polarity, of which Farraday has done so much to establish the existence in all matter, in one form or another, seems to involve that, to constitute atoms, there must be two centres of analogous, but opposite, forces in each: whence it ensues that crystals shoot in prisms or spiculæ, as water is seen to shoot in freezing; and through which salts, as deposited by the evaporation of the solvent from a solution of them, are seen to travel over the sides of the vessel; and upon which property the phenomena of electricity and magnetism appear to be dependent. How is this to be reconciled with this notion of each atom existing in a diffusible penetrable state throughout the space in which its properties prevail? Since these opposite polarities are energetic in their reciprocal polar attraction, what keeps them together, yet prevents them from so uniting as to produce neutralization?

1822. Mr. Exley's ideas, if admitted, leave no alternative but either to place a Newtonian atom within each of his concentric spheres, or to assume that nothing can have properties, or that effects can exist without causes. What is to cause a force at any mathematical point more than at any other? How, in case of a moving body, are the forces to appear successively to proceed from various centres, if there be nothing in which it is inherent, which moves and carries its forces or properties wheresoever it goes? Does not this suggestion that atoms are centres of their forces, by making the cart draw itself, force the effect to be its own cause? It is quite consistent with the Newtonian definition, that the resultant of the action of every part of a mass should comport as if it proceeded from a common centre, as does terrestrial gravitation; and of course, whether we have the Newtonian idea or that of Boscovitch, Farraday, or Exley, we have forces proceeding from centres. The great difference is that agreeably to the one these forces emanate from nothing; agreeably to the other, from something. I used to define matter to my pupils as that which has properties. In the mind, is not force distinguished from some moving power which gives it rise? Is not this distinction inevitable? and were the word force employed to designate the moving power which exercises force, would it not confound ideas, without altering the actual state of the case? Would it not impoverish language, without improving science?

Of Mundane, Ethereal, and Ponderable Matter, in their Chemical

relations.

1823. The bodies which occupy the attention of a chemist are found in one of three states-those of solidity, fluidity, and elasticity. Ice, liquid water, and steam exemplify these different states. The fact is thus illustrated, that the same chemical compound, consisting of oxygen and hy

drogen, may exist in either state, according to the temperature to which it may be subjected.

1824. Experience justifies the surmise, that scarcely any body in nature is utterly insusceptible of these three states, provided it were heated or refrigerated with an unlimited power.

1825. Beside the property of gravitation, of which the energy is inversely as the square of the distance, however great, (as when it enables the two suns, apparently forming but one-the double star, 61 Cygni (1340)—at the distance of six thousand millions of miles, to attract each other so as to revolve about their common centre of gravity,) atoms are endowed with a force called attraction of aggregation, which operates only at insensible distances, so that when brought into due proximity they unite and form a coherent mass. Again, they are endowed, as already mentioned, with chemical affinity, which varies with the kind of particles in which it exists as a property; being the characteristic by which they are distinguished one from the other.

1826. According to the doctrine which chemists have heretofore suggested for the existence of matter in the elastic or gaseous state, each aerial or gaseous atom was conceived to be enveloped in an atmosphere of fluid called caloric, resembling the ether in the self-repellent power of its constituent particles. This atmosphere has been assumed to impart to atoms which it envelopes its own inherent power of reciprocal repulsion, like that which those of the ether have. But Dalton showed that there was no repulsion between gaseous atoms when heterogeneous. Two or more such gases, hydrogen and nitrogen, for instance, being comprised in the same cavity, there would be no repulsion between the atoms of hydrogen and those of nitrogen, but only between those of the same gas. This has been held to be equally true, however many gases might be mingled, or whatever vapours might be superadded.

1827. The idea is thus refuted, which ascribes the repulsive power to the same elastic fluid, since in that case the diversity of the gaseous atoms could not so affect the repulsive influence as to nullify it between heterogeneous atoms, while sustaining this repulsion, where the atoms should be alike.

1828. Moreover, as the rays of light have been found to be mere undulations in the ether; the rays of heat, being perfectly analogous in their attributes, must also be due to ethereal undulations. But vaporization may be affected by radiant heat, and gases owe their aeriform state to the same cause as vapor or steam; yet transient undulations evidently cannot form a permanent combination, so as to confer the durable elasticity of a permanent gas.

1829. It appears, then, that neither the doctrine of caloric, nor the undulatory doctrine, as it is received, will explain the creation of permanent gas. Under these circumstances a modification of the existing opinions is

called for. It has, for some years, occurred to me, that the Newtonian doctrine of radiation might be associated with that of undulation.

1830. The fact that radiant heat could be collected by a mirror so as to raise the temperature of bodies placed in the focus, and that this process could take place in vacuo, as ascertained by Sir Humphrey Davy, had been adduced as unquestional le evidence of the materiality of caloric, the supposed fluid cause of heat. But as the cold proceeding from a snowball or any cold body could be collected by the same process, it was urged by some chemists that the evidence of the materiality of the cause of cold must also be admitted. Prevost met this argument by suggesting that no body in nature is absolutely cold. Every body, however refrigerated, is not so cold as to be incapable of greater refrigeration. Hence all bodies being absolutely above the zero of nature, are throwing off rays to each other, and where there is equality of temperature, they do not cause any change in their relative temperatures. The rays thrown off by A are compensated by those which it receives from B, and vice versa. But if A throws off to B more than B reciprocates, the temperature of A must fall until an equilibrium is attained. Thus, A being the mirror and B the snowball, the mirror is refrigerated, and causes a greater radiation from any body situated about its focus. This explanation was generally received, but to me, the following rationale, which I advanced, appeared preferable :

1831. I assumed caloric to exist throughout the sublunary creation, as the luminiferous ether is assumed to be diffused throughout all space by the undulationists; the diffusion arising from the reciprocal repulsion of its particles being similar to that which had been supposed to cause the diffusion of caloric. There is the greatest analogy between this diffusion and that which is known to exist in the case of gases. The process is the same, whether the gas be dense like chlorine, or thirty-six times as rare, as in the instance of hydrogen, and in the luminiferous ether resembles the process by which hydrogen is rarified, or might be rendered more rare, were the pressure of the atmosphere removed.

1832. It is known that in any gas or gaseous mixture like that which we breathe, if a deficit of pressure be caused in any spot, the gaseous particles will quickly move toward it, in order to restore the equilibrium of pressure, and that if, on the other hand, any augmentation of pressure be produced at any spot, the gas will move outward to restore the equilibrium.

1833. The particles being symmetrically arranged in lines, a row of particles may be conceived to lie between every two remote points. If we suppose any number of points in the focal body, and a corresponding number in the surface of the mirror, it may be conceived that the intervening ethereal or calorific particles will move in rows one way or the other, as the pressure in the focal space may become greater or less. Thus an effect is brought about, equivalent to that which the Newtonian idea of

radiation involves; lines of particles proceed from the hotter points to the colder ones.

1834. The arrangement of the particles of caloric, which was originally, in my view, confined to the sublunary creation, appears of necessity to belong to the luminiferous ether, required by the theory ascribing light to undulations, though the last-mentioned medium must be endowed with ubiquity as above stated, so as to abound in every part of space through which light reaches the eye.

1835. The undulatory hypothesis supposes that a wave-like motion being imparted to a row of particles, by a luminous point in the surface of the luminous body, is transmitted, like the sound producing waves in the air, to the other end of the row.

1836. This undulatory progression has been roughly illustrated by the transitory serpentine movements which may be made in a cord, stretched like a clothes-line between the tops of posts.

1837. In order to make this illustration elucidate the conception which I advance, we have only to suppose that the cord, instead of being attached to the post, should be drawn rapidly over pulleys, and, while thus actuated, be subjected to a cause of undulatory vibration. It may be conceived that, by this process, the ethereal particles, while performing all which the undulatory theory requires, might at the same time perform all required by that of emission and material calorific radiation. Directed upon a vaporizable liquid, the undulations might perform the part of sensible heat; the ethereal particles, successively combining, might furnish the latent heat requisite to the constitution of vapour.

1838. Agreeably to Newton, the seven colours of the spectrum are due to as many different kinds of radiant particles of various refrangibility, or susceptibility of being bent from the rectilinear path when passed through the same refracting medium.*

1839. According to the undulatory theory, the colours are caused by diversities in the undulations producing them. Retaining this feature, the last-mentioned hypothesis, as modified by myself, appears to be competent to explain the phenomena of light as well as those of vaporization, produced by calorific radiation, since not only is any vaporizing liquid subjected to the transient effect of the undulations, but also may combine with the ethereal particles as they come into contact with it.

1840. Thus modified, the rationale of the rainbow, or prismatic spectrum, would not be that the colours indicate as many varieties of original radiant particles, but that they are to be explained agreeably to the undulatory hypothesis, which ascribes them to as many varieties in the undulations, just as the notes in music are ascribed to diversities of vibration.

* Latterly, Sir David Brewster has conceived that only three elementary species of light are requisite, according to the theory of emission, to perform all the offices which Newton ascribed to seven.

1841. The ether, under this view, performs the part heretofore assigned to latent heat, by combining with solids so as to render them sasceptible of expansion, and of electrical conduction by being liable to the polarization which constitutes electricity.

1842. Sensible heat, according to this aspect, is due to the vibrations of the ethereal fluid, which is sustained by the sun, by ignition in the interior of the earth, and by chemical reaction, including combustion and respiration.

1843. The correctness of the inference, that conductors owe their conductive power to ethereal matter entering into their composition, has been insisted upon in my strictures on Farraday's speculation in some of the preceding pages. The facts admitted by this distinguished investigator of nature's laws, gave to me a basis on which to rest an argument in favour of the existence of an imponderable cause of heat and electricity in metals, which seems to me unanswerable.

1844. Agreeably to the hypothesis respecting which the preceding preparatory suggestions have been made, gasification is not due to a repulsive atmosphere of ethereal matter, severally appropriated to each ponderable constituent atom, but to an attraction for every such atom exercised by the ethereal fluid, such as water exercises toward sugar, quicklime, salt, or any soluble substance. The ether attracts the particles of certain solids, and is of course reacted upon by them. The particles thus attracted naturally distribute themselves throughout it, at symmetrical distances. Hence the law of Pettit and Dulong is verified, which, at least, holds good with all gasifiable atoms, that their capacity is inversely as their atomic weight.

1845. The atomic weights of hydrogen, nitrogen, and chlorine being severally 1, 14, 36, when associated with equal volumes of the imponderable ether, they will have still the same weight. Equal volumes will weigh the same as the atoms with which they are associated; and the capacity for heat, being directly as the volumes, will be inversely as the weights, the calculation being the same, whether ether or caloric be the imponderable principle to which they owe their gasification. By concurring with those chemists, who estimate the atoms of oxygen at 16, instead of 8, this gas will come into the same calculation.

1846. When heterogeneous gases are confined within the same cavity, that they should not react with each other is no more wonderful, than that the same mass of water may at the same time hold different substances in solution, which may add to its hydrostatic pressure though they have no reciprocal reaction.

1847. Sensible heat appears to be due to vibrations in the ether, kept up by the solar rays or central ignition within this globe. By the heat thus acquired the self-repellent power of the ether is augmented. When by refrigeration this source of repulsion is diminished beyond a certain