64 PROFESSOR W. THOMSON ON THE 



by the Sun, or exists as energy only convertible into heat by mutual actions be- 

 tween 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 cir- 

 cumstances) ; but whether intrinsically in such external matter, or developed by 

 mutual action between this matter and the Sun, and whether dynamical or poten- 

 tial in either case, requires careful consideration, as will be shewn 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 combina- 

 tion 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 ma- 

 terials 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. 



In alluding to theories of solar heat in former communications to the Royal 

 Society, I pointed out that the first hypothesis is quite untenable. In fact, it is 

 demonstrable that, unless the Sun be of matter inconceivably more conductive for 

 heat, and less volatile, than any terrestrial meteoric matter we know, he would be- 

 come dark in two or three minutes, or days, or months, or years, at his present 

 rate of emission, if he had no source of energy to draw from but primitive heat.* 

 The second has been not only held by the Fire- worshippers, but has probably been 

 conceived of by all men in all times, and considered as more or less probable by 

 every philosopher who has ever speculated on the subject. The third may have 

 occurred at any time to ingenious minds, and may have occurred and been set 

 aside as not worth considering ; but was never brought forward in any definite 

 form, so far as I am aware, until Mr Waterston communicated to the British 

 Association, during its last meeting at Hull, a remarkable speculation on cosmical 

 dynamics, in which he proposed the Theory that solar heat is produced by the im- 

 pact of meteors falling from extra-planetary space, and striking his surface with 



* This assertion is founded on the supposition that conduction is the only means by which 

 heat could reach the Sun's surface from the interior, and perhaps requires limitation. For it might 

 be supposed that, as the Sun is no doubt a melted mass, the brightness of his surface is constantly 

 refreshed by incandescent fluid rushing from below to take the place of matter falling upon the sur- 

 face after becoming somewhat cooled and consequently denser — a process which might go on for many 

 years without any sensible loss of brightness. If we consider, however, the whole annual emission 

 at the present actual rate, we find, even if the Sun's thermal capacity were as great as that of 

 an equal mass of water, that his mean temperature would be lowered by about 3° cent, in two years. 

 We may, I think, safely conclude that primitive heat within the Sun is not a sufficient source for the 

 emission which has continued without sensible (if any) abatement for 6000 years. — (May 4, 1854.) 



