410 Professor W. Thomson on the 



times as much mechanical energy* as that of the motions of all 

 the known planets taken together. The energy — that of light 

 and radiant heat — thus emitted, is dissipated always more and 

 more widely through endless space, and never has been, pro- 

 bably never can be, restored to the sun, without acts as much 

 beyond the scope of human intelligence as a creation or annihi- 

 lation of energy, or of matter itself, would be. Hence the 

 question arises. What is the source of mechanical energy, drawn 

 upon by the sun, in emitting heat, to be dissipated through 

 space ? In speculating on the answer, we may consider whether 

 the source in question consists of dynamical energy, that is, 

 energy of motion f, or of "potential energy'' (as Mr. Rankine 

 has called the energy of force acting between bodies which will 

 give way to it unless held) ; or whether it consists partly of dy- 

 namical and partly of potential energy. 



And again, we may consider whether the source in question, 

 or any part of it, is in the sun, or exists in surrounding matter, 

 until taken in and sent out again by the sun, or exists as energy 

 only convertible into heat by mutual actions between the sun 

 and surrounding matter. 



If it be dynamical and entirely in the sun, it can only be 

 primitive heat ; if potential and in the sun, it can only be energy 

 of chemical forces ready to act. If not in the sun, it must be 

 due to matter coming to the sun (for it certainly is not a mere 

 communication of motion to solar particles from external energy, 

 as such could only be effected by undulations like sound or 

 radiant heat, and we know that no such anti-radiation can be 

 experienced by a body in the sun's circumstances) ; but whether 

 intrinsically in such external matter, or developed by mutual 

 action between this matter and the sun, and whether dynamical 

 or potential in either case, requires careful consideration, as will 

 be shown in the course of this communication. We see, then, 

 that all the theories which have been yet proposed, as well as 

 every conceivable theory, must be one or other, or a combination 

 of the following three : — 



I. That the sun is a heated body, losing heat. 



II. That the heat emitted from the sun is due to chemical 

 action among materials originally belonging to his mass, or that 

 the sun is a great fire. 



III. That meteors falling into the sun give rise to the heat 

 which he emits. 



communicated to the Ro5'al Society, June 1844, and published, Phil. Mag., 

 May 1845. Joule and Scoresby "On the Powers of Electro-magnetism, 

 Steam, and Horses," Phil. Mag., June 1846. 



* Once every twenty years or so. See Table of Mechanical Energies of 

 the Solar System, appended. 



f " Actual energy," as Mr. Rankine has called it. 



