96 Antecedents of Motion, Heat, and Light. 



mensions small compared with, his present dimensions, and 

 separated from one another at comparatively great distances, 

 provided always there has been no relative motion among 

 them except what is generated by mutual gravitation. If, 

 then, the whole mass of the Sun has grown by the process 

 which, according to the author's theory (certain as regards a 

 part, whether or not it may be sufficient to account for the 

 whole, of the radiation) of solar heat, we know to be augment- 

 ing it at present, there must have been generated in the whole 

 process of conglomeration the quantity of heat stated above, a 

 quantity which amounts to about 20,000,000 times as much 

 as is at present radiated off in one year. The author gave 

 reasons for believing that this heat has probably been nearly 

 all radiated off immediately on being generated ; and that 

 enough of it has not been retained in the conglomerated mass 

 to be the store from which the heat at present radiated is 

 drawn. 



That the present solar radiation is supplied chiefly from a store 

 of heat contained in the mass, whether created there or generated 

 mechanically by the impacts of meteors which have fallen in 

 during remote periods of past time, appears very improbable. 



On the contrary, there must in all probability be some agency 

 continually supplying heat to compensate the loss constantly 

 experienced by radiation from the Sun ; and that agency,* as 

 the author has shown elsewhere, can be no other than the me- 

 chanical action of masses coming from a state of very rapid 

 motion round the Sun, to rest on his surface. 



* It is quite certain that it cannot, as the nebular theory has led some to 

 suppose it may, be the energy of gravitation effecting any continued condensa- 

 tion of the Sun's present mass, since without increased pressure, it is only by 

 cooling that any condensation can be taking place ; and the heat emitted in 

 consequence of condensation by cooling, would depend merely on the specific 

 heat of the whole mass in its actual circumstances of temperature and pressure, 

 and might, (for all we know of the properties of matter at such high tempera- 

 tures and pressures) be greater than equal to, or less than, the thermal equiva- 

 lent of the work done by gravity on the contracting body. Thus the heat 

 emitted by a mass of air, contracting under any constant pressure, is greater 

 than the amount mechanically equivalent to the work done by the pressure. 

 The heat emitted by a mass of water, or of mercury, cooling from 100° to 50° 

 Cent, under any constant pressure exceeding about 90,000 atmospheres, is less 

 than the amount of heat mechanically equivalent to the work done by the pres- 

 sure on the contracting mass. 



