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 hydrogen, 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: