TRANSACTIONS OF THE SECTIONS. 2? 



each other through intei'vening molecules of the same kind, and the qualities of 

 matter as dependmg on the motions of different orders of molecules. In pm'Suance 

 of the same views, magnetic attractions and repulsions were to be referred to differ- 

 ential conditions of pressiu-e. 



Physical Considerations regarding the Possible Age of the Sun's Heat. 

 By Professor W. Thomson, F.R.S. 



The author prefaced his remarks by di-awing attention to some principles previously 

 established. It is a principle of iiTeversible action in natiu-e, that, " although me- 

 chanical energy is indestructible, there is a universal tendency to its dissipation, 

 which produces gradual augmentation and diffusion of heat, cessation of motion, and 

 exhaustion of potential energy, thi-ough the material universe." The result of this 

 would be a state of imiversal rest and death, if the universe were finite and left to 

 obey existing laws. But as no limit is known to the extent of matter, science 

 points rather to an endless progi-ess through an endless space, of action involving 

 the transformation of potential energy thi-ough palpable motion into heat, than to 

 a single finite mechanism, rimning down like a clock and stopping for ever. It is 

 also impossible to conceive either the beginning or the continuance of life without a 

 creating and oven'uling power. The author's object was to lay before the Section 

 an application of these general views to the discover}' of probable limits to the 

 periods of time, pas< anifufure, dimng which the sun can be reckoned on as a source 

 of heat and light. The subject was divided under two heads : 1. On the secular 

 cooling of the sun ; 2. On the origin and total amount of the sim's heat. 



In the first part it is shown that the sun is probably an incandescent liquid mass, 

 radiating away heat without any appreciable compensation by the influx of meteoric 

 matter. The rate at which heat is radiated from the sun has been measm-ed by 

 Herschel and Pouillet independently ; and, according to then- residts, the author 

 estimates that if the mean specific heat of the sim were the same as that of liquid 

 water, his temperature would be lowered by 1°'4 Centigrade annually. In con- 

 sideiiug what the sun's specific heat maj' actually be, the author first remarks that 

 there are excellent reasons for believing that his su.bstance is very much like the 

 eai-th's. For the last eight or nine years, Stokes's Principles of Solar and Stellar 

 Chemistry have been taught in the public lectures on natiu'al philosophy in the 

 University of Glasgow ; and it has been shown as a first result, that there certainly 

 is sodivm in the sun's atmosphere. The recent application of these principles in the 

 splendid researches of Bunsen and Kirchhoff (who made an independent discovery 

 of Stokes's theory) has demonstrated with equal certainty that there are iron and 

 manganese, and several of our other known metals, in the sun. The specific heat of 

 each of these substances is less than the specific heat of water, which indeed 

 exceeds that of every other known ten'estrial solid or liquid. It might therefore 

 at first sight seem probable that the mean specific heat of the sun's whole substance 

 is less, and veiy certain that it cannot be much greater, than that of water. But 

 thermodynamic reasons, explained in the paper, lead to a very different conclusion, 

 and make it probable that, on account of the enormous pressure which the sim's 

 interior bears, his specific heat is more than ten times, although not more than 

 10,000 times, that of liquid water. Hence it is probable that the sim cools by as 

 much as 14° C. in some time more than 100 years, but less than 100,000 years. 



As to the sun's actual temperature at the present time, it is remarked that at his 

 surface it cannot, as we have many reasons for believing, be incomparably higher 

 than temperatures attainable artificially at the earth's surface. Among other rea- 

 sons, it may be mentioned that he radiates heat from every square foot of his sur- 

 face at only about 7000 horse-power. Coal burning at the rate of a little less than 

 a pound per two seconds would generate the same amount ; and it is estimated 

 (Rankine, 'Prime Movers,' p. 285, edit. 1859) that in the furnaces of locomotive 

 engines, coal bums at from 1 lb. in 30 seconds to 1 lb. in 90 seconds per square foot 

 of grate-bars. Hence heat is radiated from the sim at a rate not more than from 

 fifteen to foi-ty-five times as high as that at which heat is generated on the grate- 

 bars of a locomotive fm-nace, per equal areas. 



The interior temperature of the sun is probably far higher. than that at the sur- 

 face, because conduction can play no sensible part in the transference of heat between 



