emit air-particles, constituting a thin, elastic fluid, of great efficacy in performing the ordinary operations of nature, it seems not unnatural to expect, that the small particles of bodies should emit a proportionally attenuated air; i.e. an æther which may likewise have a great share in the subtle actions of the small particles of bodies over each other. The emission of odoriferous particles, light, magnetical and electrical effluvia, may also be some presumption in favour of the existence of the æther. Moreover, it is reasonable to expect that it should have a repulsive force in respect of the bodies which emit it; and for the same reasons its particles may repel each other. It may therefore be elastic, compressible, and apt to receive vibrations from the last cause; and from the first may be rarer within the pores of bodies than in large open spaces, and grow denser as the distance from gross matter increases. Our air is indeed denser near the earth than in the higher regions; but this is owing to its gravity prevailing against its expansive force. If we suppose the gravity of the æther to be very small, and its elasticity or expansive and repulsive force very great, both which must be supposed, if we admit it at all in the manner proposed by Sir Isaac Newton, its density may increase in receding from gross matter, and be much less in the pores of bodies than in open spaces void of gross matter. Thus we may suppose even the air, which remains in the large pores of such bodies as repel its particles, to be rarer than the common external air.
Lastly, let us suppose the existence of the æther, with these its properties, to be destitute of all direct evidence, still if it serve to explain and account for a great variety of phænomena, it will have an indirect evidence in its favour by these means. Thus we admit the key of a cypher to be a true one, when it explains the cypher completely; and the decypherer judges himself to approach to the true key, in proportion as he advances in the explanation of the cypher; and this without any direct evidence at all. And as the false and imperfect keys, which turn up to the decypherer in his researches, prepare the way for the discovery of the true and complete one, so any hypothesis that has so much plausibility as to explain a considerable number of facts, helps us to digest these facts in proper order, to bring new ones to light, and to make experimenta crucis for the sake of future inquirers. The rule of false affords an obvious and strong instance of the possibility of being led, with precision and certainty, to a true conclusion from a false position; and it is of the very essence of algebra to proceed in the way of supposition.
We come next to consider the uniformity and continuity of the white medullary substance of the brain, spinal marrow, and nerves. Now these are evident to the eye, as far as that can be a judge of them. The white medullary substance appears to be every where uniform and similar to itself throughout the whole brain, spinal marrow, and nerves; and though the cortical
