70 PROFESSOR W. THOMSON ON THE 



fering more absorption from its atmosphere of transparent gaseous products* than 

 the light of the Sun actually does experience from the dense atmosphere through 

 which it passes. Let us therefore consider at what rate such a body, giving out 

 heat so copiously, would diminish by burning away. The heat of combustion 

 could probably not be so much as 4000 thermal units per pound of matter burned,f 

 the greatest thermal equivalent of chemical action yet ascertained falling con- 

 siderably short of this. But 2781 thermal units (as found above) are emitted per 

 second from each square foot of the Sun ; hence there would be a loss of about 

 •7 of a pound of matter per square foot per second. Such a loss of matter from 

 every square foot, if of the mean density of the Sun (a little more than that of 

 water), would take off from the mass a layer of about "5 of a foot thick in a 

 minute, or of about 55 miles thick in a year. At the same rate continued, a 

 mass as large as the Sun is at present would burn away in 8000 years. If the 

 Sun has been burning at that rate in past time, he must have been of double 

 diameter, of quadruple heating power, and of eight-fold mass, only 8000 years 

 ago. We may quite safely conclude then that the Sun does not get its heat by 

 chemical action among particles of matter primitively belonging to his own mass, 

 and we must therefore look to the meteoric theory for fuel, even if we retain the 

 idea of a fire. Now, according to Andrews, the heat of combustion of a pound 

 of iron in oxygen gas is 1301 thermal units, and of a pound of potassium in chlo- 

 rine 2655 ; a pound of potassium in oxygen 1 700 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 6000th part, ac- 

 cording to Mr Wateeston's theory, and, according to the form of the Gravitation 

 Theory now proposed, only the 3000th part, of the least amount of dynamical 

 energy a meteor can have on entering the region of ignition in the Sun's atmo- 

 sphere. Hence a mass of carbon entering the Sun's atmosphere, and there burn- 

 ning with oxygen, could only by combustion give out heat equal to the 3000th 

 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 i *oo 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. 

 Probably 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 com- 

 bustion alone) only i^ m part of the heat and light of a mass of iron entering the 



* These would rise and be regularly diffused into space. 



f Both the elements that enter into combination are of course included in the weight of the burn- 

 ing matter. 



