EQUILIBRATION. 507 



rapid diffusion, must, as the concentration approaches its 

 limit, diminish in amount, and eventually leave only an in- 

 appreciable internal remnant. With or with- 

 out the accompaniment of that hypothesis of nebular 

 condensation, whence, as we see, it naturally follows, 

 the doctrine that the Sun is gradually losing his heat, 

 has now gained considerable currency; and calcula- 

 tions have been made, both respecting the amount 

 of heat and light already radiated, as compared with 

 the amount that remains, and respecting the period during 

 which active radiation is likely to continue. Prof. Helm- 

 holtz estimates, that since the time when, according to the 

 nebular hypothesis, the matter composing the Solar System 

 extended to the orbit of Neptune, there has been evolved 

 by the arrest of sensible motion, an amount of heat 454 

 times as great as that which the Sun still has to give out. 

 He also makes an approximate estimate of the rate at which 

 this remaining ^-^th is being diffused: showing that a 

 diminution of the Sun's diameter to the extent of 10.000? 

 would produce heat, at the present rate, for more than 2000 

 years; or in other words, that a contraction of ^-uvjnnr.Trinr 

 of his diameter, suffices to generate the amount of light 

 and heat annually emitted ; and that thus, at the present rate 

 of expenditure, the Sun's diameter will diminish by some- 

 thing like -fa in the lapse of the next million years.* Of 

 course these conclusions are not to be considered as more 

 than rude approximations to the truth. Until quite recent- 

 ly, we have been totally ignorant of the Sun's chemical 

 composition; and even now have obtained but a superficial 

 knowledge of it. We know nothing of his internal struc- 

 ture; and it is quite possible (probable, I believe,) that the 

 assumptions respecting central density, made in the forego- 

 ing estimates, are wrong. But no uncertainty in the data on 



* See paper " On the Inter-action of Natural Forces," by Prof. Helmholtz, 

 translated by Prof. Tyndall, and published in the Philosophical Magazine, 

 supplement to Vol. XI. fourth series. 



