370 "Mr. J. Croll on Geological Time, and the probable 



is quite conceivable that this nebulous mass might have been 

 possessed of an original store of heat previous to condensation. 

 It is quite possible that the very reason why it existed in such 

 a rarefied or gaseous condition was its excessive temperature, and 

 that condensation only began to take place as the mass began 

 to cool down. It seems far more probable that this should have 

 been the case than that the mass existed in so rarefied a condition 

 without temperature. For why should the particles have existed 

 in this separated form when devoid of the repulsive energy of 

 heat, seeing that in virtue of gravitation they had such a ten- 

 dency to approach to one another ? But if the mass was origi- 

 nally in a heated condition, then in condensing it would have to 

 part not only with the heat generated in condensing, but also 

 with the heat which it originally possessed, a quantity which 

 would no doubt much exceed that produced by condensa- 

 tion. To illustrate this principle, let as suppose a pound of air, 

 for example, to be placed in a cylinder and heat applied to it. 

 If the piston be so fixed that it cannot move, 234*5 foot-pounds 

 of heat will raise the temperature of the air 1° C. But if the 

 piston be allowed to rise as the heat is applied, then it will re- 

 quire 330*2 foot-pounds of heat to raise the temperature 1° C. 

 It requires 95*7 foot-pounds more heat in the latter case than 

 in the former. The same amount of energy, viz. 234' 5 foot- 

 pounds, in both cases goes to produce temperature ; but in the 

 latter case, where the piston is allowed to move, 95*7 foot-pounds 

 of additional heat is consumed in the mechanical work of raising 

 the piston. Suppose, now, that the air is allowed to cool under 

 the same conditions : in the one case 234*5 foot-pounds of heat 

 will be given out while the temperature of the air sinks 1° C. ; 

 in the other case, where the piston is allowed to descend, 330*2 

 foot-pounds will be given out while the temperature sinks 1° C. 

 In the former case, the air in cooling has simply to part with 

 the energy which it possesses in the form of temperature ; but 

 in the latter case it has, in addition to this, to part with the 

 energy bestowed upon its molecules by the descending piston. 

 While the temperature of the gas is sinking 1°, 95*7 foot-pounds 

 of energy in the form of heat is being imparted to it by the de- 

 scending piston ; and this has to be got rid of before the tempe- 

 rature is lowered by 1°. Consequently 234-5 foot-pounds of 

 the heat given out previously existed in the air under the form 

 of temperature, and the remaining 95*7 foot-pounds given out 

 was imparted to the air by the descending piston while the gas 

 was losing its temperature. 234*5 foot-pounds is the energy 

 or heat which the air previously possessed, and 95*7 is the energy 

 or heat of condensation. 



In the case of the cooling of the sun from a nebulous mass, 



