Passing over a few names of less note in this connection, (Berthier, Churcol, and others,) we find a somewhat more developed attempt at unveiling the standing enigma, presented in the writings of John Herapath, of Bristol, England. In a preliminary essay, published in Thomson's Annals of Philosophy, "On the Physical Properties of Gases," he announced the hypothesis of "one cause for heat, light, gravitation, electricity, cohesion, aerial repulsion, &c., from which all these flow, and are easily deducible ; and their effects may be computed by mathematical induction, [deduction ?] It shows us that gravitation, cohesion, and affinity, are but the same thing under different modifications; that the differences of the two latter arise from a difference in the figures and sizes only of the particles; that attraction and repulsion are not properties of matter." This hypothesis thus briefly stated by its author, at the age of twenty-three years, does not appear to have been further publicly elaborated for five years.
In 1821, Herapath contributed to the same journal a memoir entitled "A Mathematical Inquiry into the Causes, Laws, and principal Phenomena of Heat, Gases, Gravitation, &c.," memorable as presenting by far the fullest and clearest exposition of the mechanical theory of heat and of gases that had at that time been propounded. He states that about ten years earlier, while engaged in investigating an anomaly found in his calculations of lunar equation, his attention had been accidentally directed to Newton's suggestions as to the cause of gravitation ; and he proceeds :
"If gravitation depends upon the action of an elastic medium such as Newton supposed, which grows rarer aud rarer as you approach the dense bodies of the sun and planets, there ought to be some reason for this variation of density ; and as Newton has not, as far as I could perceive, given any, I began to consider what it might be. And after some little thinking, it occurred to me that if this medium be of the same nature as our atmosphere and other gaseous bodies, that is, if it be capable of being expanded by heat and contracted by cold, then the sun being a very hot body, and the heat being so much the greater the nearer we are to him, the density of the medium ought therefore to decrease with a decreasing and increase with an increasing distance, the same as Newton would have it. And because we find by experience that dense solid bodies receive heat more strongly than much rarer [224] ones, particularly than gases, the dense bodies of the planets being heated by the solar rays as well as by the medium about them, ought it appeared to me, to be hotter than this medium, and consequently ought to produce the same effects on the medium as the sun, though cot in so great a degree. Therefore if as Newton imagines, the particles of the planets be impelled toward the sun by the inequality of the pressure on their further and nearer sides, the denser parts of the medium pressing more forcibly than the rarer, the same reason will likewise hold good why bodies should be impelled toward the planets an<l other material parts of the system."