Mechanical Energies of the Solar System. 417 



liquid (as they might be wholly or partly if the fuel be metallic) 

 they would interfere with the supply of the elements from below. 

 In either or in both ways the fire would be choked^ and I think 

 it may be safely affirmed that no such fire could keep alight for 

 more than a few minutes, by any conceivable adaptation of air 

 and fuel. If then the sun be a burning mass, it must be more 

 analogous to burning gunpowder than to a fire burning in air ; 

 and it is quite conceivable that a solid mass, containing within 

 itself all the elements required for combustion, provided the pro- 

 ducts of combustion are permanently gaseous^, could burn off at 

 its surface all round, and actually emit heat as copiously as the 

 sun. Thus an enormous globe of gun-cotton might, if at first 

 cold, and once set on fire round its surface, get to a permanent 

 rate of burning, in which any internal part would become heated 

 by conduction, sufficiently to ignite only when nearly approached 

 by the diminishing surface. It is highly probable indeed that 

 such a body might for a time be as large as the sun, and give 

 out luminous heat as copiously, to be freely radiated into space, 

 without suffering more absorption from its atmosphere of trans- 

 parent gaseous products t 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 J, the greatest thermal 

 equivalent of chemical action yet ascertained falling considerably 

 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 



* On this account gunpowder would not do. 

 \ These would rise and be regularly diffused into space. 

 X Both the elements that enter into combination are of course included 

 in the weight of the burning matter. 



