418 Professor W. Thomson on the 



combustion of a pound of iron in oxygen gas is 1301 thermal 

 units, and of a pound of potassium in chlorine 2655 ; a pound 

 of potassium in oxygen 1700 according to Joule; and carbon in 

 oxygen, according to various observers, 8000. The greatest of 

 these numbers, multiplied by 1390 to reduce to foot-pounds, 

 expresses only the 6000dth part, according to Mr. Waterston's 

 theory, and, according to the form of the gravitation theory 

 now proposed, only the 3000dth part, of the least amount of 

 dynamical energy a meteor can have on entering the region of 

 ignition in the sun's atmosphere. Hence a mass of carbon 

 entering the sun's atmosphere, and there burning with oxygen, 

 could only by combustion give out heat equal to the 3000dth part 

 of the heat it cannot but give out from its motion. Probably no 

 kind of known matter (and no meteors reaching the earth have 

 yet brought us decidedly new elements) entering the sun's 

 atmosphere from space, whatever may be its chemical nature, 

 and whatever its dynamical antecedents, could emit by combus- 

 tion as much as ^ ^'^^dth of the heat inevitably generated from its 

 motion. It is highly probable that many, if not all, meteors 

 entering the sun's atmosphere do burn, or enter into some 

 chemical combination with substances which they meet. Pro- 

 bably meteoric iron comes to the sun in enormous quantities, 

 and burns in his atmosphere, just as it does to the earth. But 

 (while probably nearly all the heat and light of the sparks which 

 fly from a steel struck by a flint is due to combustion alone) only 

 y^i^odth part of the heat and light of a mass of iron entering 

 the sun's atmosphere, or ^th of the heat and light of such a 

 meteor entering our own, can possibly be due to combustion. 

 Hence the combustion of meteors may be quite disregarded as a 

 source of solar heat. 



At the commencement of this communication, it was shown 

 that the heat radiated from the sun is either taken from a stock 

 of primitive solar heat, or generated by chemical action among 

 materials originally belonging to his mass, or due to meteors 

 falling in from surrounding space. We saw that there are suf- 

 ficient reasons for utterly rejecting the first hypothesis ; we have 

 now proved that the second is untenable; and we may con- 

 sequently conclude that the third is true, or that meteors falling 

 in from space give rise to the heat which is continually radiated 

 off by the sun. We have also seen that no appreciable portion 

 of the heat thus produced is due to chemical action, either between 

 the meteors and substances which they meet at the sun, or 

 among elements of the meteors themselves ; and that whatever 

 may have been their original positions or motions relatively to 

 one another or to the sun, the greater part of them fall in 

 gradually from a state of approximately circular motion, and 



