372 Mr. J. Gill on the Dynamical Theory of Heat. 



and permanence which naturally present themselves to the unpre- 

 judiced mind in contemplating these sublime phenomena. Now 

 by an interesting coincidence it appears that, by supposing the 

 meteoric theory so modified as to avoid the foregoing objection, 

 it would result from this modification that terrestrial molecular 

 motion may, in agreement with known physical laws, be con- 

 verted into the force of gravitation by the agencies of the meteoric 

 phenomena, thus removing a strong objection which would other- 

 wise oppose the repulsion theory of thermo-dynamics which I am 

 endeavouring to develope, while at the same time it supports the 

 idea of a general cosmical stability and permanence. In specu- 

 lations of this kind we cannot expect to arrive at conclusions 

 beyond a mere preponderance of probability on one side or the 

 other of any undecided point ; and in such investigations the 

 ardent study of the limited train of facts we possess inevitably 

 leads into the regions of the unknown ; for the vast field of 

 research offers no boundaries ; no one part of the subject can be 

 fairly detached from the apparently endless circuit of connexions 

 and relations with which it is bound ; and in directing our ex- 

 perimental researches beyond the narrow limits of established 

 fact in search of additional truth, it were unwise to reject the 

 suggestions of analogical probability. 



Radiant heat is supposed to be propagated in the same manner 

 as light. All bodies radiate heat, and every point of the surface 

 of each is a distinct centre from which undulations of the uni- 

 versal sether proceed in straight lines in all directions without 

 interfering with each other, or with the innumerable waves with 

 which space is filled. Heat in common terrestrial matter has 

 been well designated a mode of motion ; but it must be conceived 

 of as molecular motion of a vibratory or orbital character, limited 

 by the balance of forces affecting the particles of a mass under 

 given circumstances. No uninterrupted independent rectilinear 

 motion, either in an atom or a mass, or in the stright-line trans- 

 mission of radiation or electricity, can be imagined of as heat, 

 but rather as vis viva, which may become heat when the motion 

 of a body is stopped and transformed into individual movements 

 of its own particles, or of those of other matter. 



It is allowed that the chief physical source of terrestrial energy 

 is the sun. The wonderful balance of the perturbations of the 

 solar system, as shown by Laplace and other astronomers, and 

 the rational probability of the immense antiquity of the earth's 

 existence indicated by geological evidence, suggest ideas of the 

 measure of time in cosmical existence somewhat in agreement 

 with the overwhelming measures of quantity and extension in 

 the universe which even the limited powers of man can faintly 

 appreciate. All things finite must have had a beginning, and 



