KINETIC THEORIES OF GRAVITATION. 223 



ing questions " are suggested by the hypothesis of retber pressure, as by 

 the hypothesis ol" an original "proi)ensity to approach." 



The speculation however, is too indeterminate to admit of precise 

 criticism ; and is noteworthy only from the eminence of its proposer. 

 Jt is a little remarkable that Euler, although in correspondence with 

 Lesage, makes no allusion to his hypothesis. 



Herapath. 1816. 



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 Her- 

 apath, of Bristol, England. In a preliminary essay, published in Thom- 

 son's Annals of Philosopliy, " 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 mathe- 

 matical 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 onl}' of the particles; that attraction and repulsion are not 

 properties of matter."* This hypothesis thus briefly stated by its au- 

 thor, at the age of twenty-three years, does not appear to have been fur- 

 ther 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 Phenom- 

 ena 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 npon 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 j)er- 

 ceive, 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 de- 

 crease 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 



* Annals of Philosophy, July, 1816, vol. viii, pp. 58, 59. 



