which causes the Stellar Radiations. 223 



by the chemical force. A few words more are necessary on 

 the source of solar heat. My paper opposing the mechanical 

 theories of solar heat, the fall of meteorites in the sun, and the 

 fall of the materials of the sun towards its centre, was pub- 

 lished in the ' Proceedings,' for 1867, of the Academy of 

 Natural Sciences of Philadelphia ; and at its close I expressed 

 a hope that the subject was put at rest for ever. But Mr. 

 Croll has lately brought out an addition to the mechanical 

 theory. He admits the failure of the meteoric theory; and he 

 concedes that the falling together by gravity of the materials 

 of the sun would produce heat enough for only twenty millions 

 of years — an insufficient period. But he supposes that two 

 unluminous stars, each half the mass of our sun, must have 

 been propelled together in opposite directions with velocities 

 of 476 miles per second, and that their collision must have 

 produced heat enough to supply the solar radiations for 50 

 millions of years. This vast amount of heat would have ex- 

 panded our sun very widely in a nebulous condition ; and by 

 the falling together of this nebulous mass, heat enough would 

 be produced to last 20 millions of years more of solar radia- 

 tions. These 50 millions of years of heat derived from the 

 collision, and these 20 millions derived from the falling together 

 of the solar materials, make together 70 millions of years of 

 solar light and heat. Many objections oppose all this. 



1. If the solar heat for 20 millions of years would be pro- 

 duced by the falling together of the materials of the sun, then 

 precisely the same amount of energy, or heat, would be con- 

 sumed to expand the sun to its nebulous condition. And this 

 would be subtracted from the heat of collision, 50 millions of 

 years. Instead therefore of 70 millions of years, his period 

 would remain only the original 50 millions from collision. 



2. Mr. Croll has not proved, nor attempted to prove, that 

 the heat of fifty millions of years would be sufficient to expand 

 our sun far beyond the furthest planet, so that in condensing 

 the solar system might be formed. When expanded only to 

 the orbit of Neptune the sun was 14,000,000 less dense than 

 hydrogen. By the Table of Marsh this would absorb and 

 render latent a wonderful amount of latent heat, which Mr. 

 Croll must prove could be produced by his theoretical collision. 



3. If our Sun were expanded beyond the orbit of Neptune 

 by a sudden production of heat from any cause, then, in order 

 to contract to nearly his present size, that heat must first ra- 

 diate away. According to this scheme, the very fact of solar 

 condensation presupposes the loss of heat by radiation. But 

 after the loss of all this heat which caused the expansion (the 

 heat of 50 millions of years), none at all can be left for solar 



