ment of the letter, weight, in its usual practical sense, is only one case of the general force which causes all ponderable masses of matter to gravitate toward each other, and which is of course liable to resist any conflicting motion, whatever may be the direction. When, in the form of solar attraction, it overcomes that inertia of the planets which would otherwise cause them to leave their orbits, does gravitation “resist motion upward ?” 1813. In the next paragraph you allege, that "there is a difference in these two kinds of resistance to motion. Inertia is instantaneous, weight is continuous, resistance." 1814. It is to this allegation I object, that as you have defined inertia to be "resistance to motion, or to change of motion," it follows that it can be instantaneous only where the impulse which it resists is instantaneous. It cannot be less continuous than the force by which it is overcome. 1815. Gravity has been considered as acting upon falling bodies by an infinity of impulses, each producing an adequate acceleration; but to every such accelerating impulse, producing of course a "change of motion,' will there not be a commensurate resistance from inertia? and the impulses and resistances being both infinite, will not one be as continuous as the other? 1816. I have already adverted to inertia as the continuous antagonist of solar attraction in the case of revolving planets. 1817. Agreeably to Mossotti, the creation consists of two kinds of matter, of which the homogeneous particles are mutually repellent, the heterogeneous mutually attractive. Consistently with this hypothesis, per se, any matter must be imponderable; being endowed with a property the very opposite of attraction of gravitation.. This last-mentioned property exists between masses consisting of both kinds of particles, so far as the attraction between the heterogeneous atoms predominates over the repulsion between those which are homogeneous. It would follow from these premises, that all matter is ponderable or otherwise, accordingly as it may be situated. 1818. Can the ether by which, according to the undulatory theory, light is transmitted, consist of ponderable matter? Were it so, would it not be attracted about the planets with forces proportioned to their weight, respectively? and becoming of unequal density, would not the diversity in its density, thus arising, affect its undulations, as the transmission of sound is influenced by any variations in the density of the aeriform fluid by which it is propagated? With esteem, I am yours truly, (See appendix for Whewell's Essay.) Additional Remarks on the Speculations of Farraday and Exley, above noticed. 1819. Is it possible for a mere centre to be endowed with a force? or reasonable that language should not make a distinction between something and nothing, between cause and effect, between matter and the properties of matter? m being the properties, and a the Newtonian atom, of which they have been considered as the attributes, I cannot concur in the reasoning which infers that where we can only perceive phenomena, we are to dispense with the idea of causation, because that causation is not directly perceptible. It seems to me, from the meaning of the words, that no cause can exist without some effect, nor can any effect exist without a cause. Language founded on the existence of ideas cannot be disused. Can there be any reason for considering any thing as endowed with existence which gives no evidence of existence? We distinguish between the thing which causes and the effect which it produces. The cause evidently has a centrality; the effect, though it indicates by the direction in which it arrives, the centre whence it proceeds, is remote from that centre. The existence of this centrality seems to be recognised in the suggestion that atoms are centres of forces. This implies that the source or cause is at the centre in each atom, and, of course, the phenomenon, being more or less remote from the centre, cannot be the source or cause, and hence has been treated as an effect or property. 1820. The suggestion that the office of atoms may be performed by centres of forces, in fact, assigns to a mere centre the part now performed by a Newtonian atom. But it must be evident that the centre is that point within any rotating mass, which does not turn therewith; and which, where neither of the opposite motions resulting from rotation take place, can neither have length nor breadth. This reduces the idea of a centre to a common definition with a mathematical point; which is nihility in the extreme. An absolutely void space may be identified with nihility, and a mathematical point is a portion of that space, without length, breadth, or thickness. To endow centres with forces is to disregard the axiom, "Out of nothing nothing can come." Moreover, wherefore should there be a force at certain mathematical points, and yet others be destitute of the same attribute? Manifestly, if some mathematical points are deficient of powers with which others are endowed, there must be something associated with one, which is not associated with the other. This justifies the Newtonian idea, that the force, though proceeding from the centre, is, like the terrestrial attraction of gravitation, the resultant of the complicated attraction of the whole of a body surrounding the centre. But the centrality of the force does not seem to accord with the idea of the inferred diffusion of properties. In the instance of gravitation it does not account for those attributes by which this globe acts as a solid mass within its 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 unquestionable 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 |