158 C. Lloyd Morgan — Geological Time. 



expect, is one which accounts for this heat by combustion, such 

 as is carried on when we bum coal in a fire-grate. But " to main- 

 tain the present rate of radiation," says Mr, Croll, "it would require 

 the combustion of about 1500 pounds of coal per hour on every 

 square foot of the sun's surface, and were the sun composed of that 

 material, it would be all consumed in less than five thousand years." 

 " The opinion," he adds, " that the sun's heat is maintained by com- 

 bustion, cannot be entertained for a single moment." 



Another hypothesis shall be stated and answered in the words of 

 Professor Tyndall (Heat a Mode of Motion, p. 478): "The sun 

 we know rotates upon his axis once in about twenty-five days ; and 

 the notion has been entertained that the friction of the periphery 

 of this wheel against something in surrounding space produces 

 the light and heat. But what forms the brake, and by what 

 agency is it held, while it rubs against the sun ? Granting, more- 

 over, the existence of the brake, we can calculate the total amount 

 of heat which the sun could generate by such friction. We know 

 his mass ; we know his time of rotation ; we know the mechanical 

 equivalent of heat ; and from these data we can deduce, with 

 certainty, that the force of rotation, if entirely converted into heat, 

 would cover less than two centuries of emission." The Gravitation 

 Theory is, indeed, the only hypothesis which is generally admitted 

 by physicists, but of that theory there are two forms. One of 

 these, which is known as the Meteoric Theory, was first proposed 

 by Dr. Meyer, of Heilbronn, According to this hypothesis, the 

 sun's heat is maintained by the continual fall upon his surface 

 of meteoric matter, by a constant rain of meteorites. The amount 

 of energy exerted by one pound weight falling upon the sun from 

 an infinite distance would be sufficient to raise, says Mr. Croll, 

 one thousand tons to a height of five and a half miles. " It would 

 project the Warrior, fully equipped with guns, stores, and ammuni- 

 tion, over the top of Ben Nevis." '• Prodigious as is the energy 

 of a single pound of matter falling into the sun, nevertheless a 

 range of mountains, consisting of a hundred and seventy-six cubic 

 miles of solid rock, falling into the sun, would maintain his heat 

 for only a single second. A mass equal to that of the earth would 

 maintain the heat for only ninety-three years, and a mass equal 

 to that of the sun itself falling into the sun, would afford but 

 thirty-three million years' sun heat." The Meteoric Theory has now 

 given way before that known as the Contraction or Condensation 

 Theory of Helmholtz. 



If we take a closed cylinder, in which works a closely-fitting piston, 

 and, by a sudden downward motion of the piston, condense the air 

 within the cylinder, we shall raise the temperature of that air, and 

 this to such an extent, that a piece of German tinder, placed at the 

 bottom of the cylinder, may be ignited in this way. If the pressure 

 were to be brought to bear slowly instead of rapidly, an equal 

 amount of heat would be produced, and if, instead of the external 

 force of muscular energy, the internal force of the gravitation of the 

 particles of the gaseous air towards each other were to cause the 



