2 Sir William Thomson [Jan. 21, 



possibly a higher, certainly not much lower, rate for a few million 

 years. How is this to be explained ? Natural philosophy cannot evade 

 the question, and no physicist who is not engaged in trying to answer 

 it can have any other justification than that his whole working time 

 is occupied with work on some other subject or subjects of his 

 province by which he has more hope of being able to advance science. 



It may be taken as an established result of scientific inquiry that 

 the sun is not a burning fire, and is merely a white hot fluid mass 

 cooling, with some little accession of fresh energy by meteors 

 occasionally falling in, but of very small account in comijarison with 

 the whole energy of heat which he gives out from year to year. Helm- 

 holtz's form of the meteoric theory of the origin of the sun's heat, 

 may be accepted as having the highest degree of scientific probability 

 that can be assigned to any assumption regarding actions of prehis- 

 toric times. The essential principle of the explanation is this ; at 

 some period of time, long past, the sun's initial heat was generated 

 by the collision of pieces of matter gravitationally attracted together 

 from distant space to build up his present mass ; and shrinkage due 

 to cooling gives, through the work done by the mutual gravitation of 

 all parts of the shrinking mass, the vast heat-storage capacity in 

 virtue of which the cooling has been, and continues to be, so slow. 



In some otherwise excellent books it is " paradoxically " stated 

 that the sun is becoming hotter because of the condensation.* Para- 

 doxes have no place in science. Their removal is the substitution of 

 true for false statements and thoughts, not always so easily 

 effected as in the present case. The truth is, that it is because the 

 sun is becoming less hot in places of equal density that his 

 mass is allowed to yield gradually under the condensing tendency 

 of gravity and thus from age to age cooling and condensation go on 

 together. 



An essential detail of Helmholtz's theory of solar heat is that the 

 sun must be fluid, because even though given at any moment hot 

 enough from the surface to any depth, however great, inwards, to be 

 brilliantly incandescent, the conduction of heat from within through 

 solid matter of even the highest conducting quality known to us, 

 would not suffice to maintain the incandescence of the surface for 

 more than a few hours, after which all would be darkness. Observa- ^ 

 tion confirms this conclusion so far as the outward appearance of the 



* [Note of February 21, 1887. — The " paradox " referred to here, is, as I now 

 find, merely a mis-statement (faulty and manifestly paradoxical through the 

 omission of an essential condition) of an astonishing and most important con- 

 clusion of a paper by J. Homer Lane, which appeared iu the ' American Journal 

 of Science, for July 1870 (referred to more particularly on p. 11 below). In 

 Newcomb's ' Popular Astronomy,' first edition, p. 508, the omission is supplied 

 in a footnote, giving a clear popular explanation of the dynamics of Lane's con- 

 clusion; and the subject is similarly explained in Ball's ' Story of the Heavens,' 

 pp. 501, 502, and 503, with complete avoidance of the " paradox." And now I 

 take this opportunity of correcting my hasty correction of the " paradox " by the 

 insertion of the five words in italics added to line 6 of the paragraph. — W. T.] 



