240 THE AGE OF THE EARTH AS AN ABODE FITTED FOR LIFE. 



distance. As the distance between any two particles is halved, their 

 mutual attraction is raised fourfold. Perpetual halving would cause 

 the attraction to mount up toward infinity. 



In the sun, then, there seems to be this interesting combination: 

 (1) A gaseous mass already above the critical temperature growing 

 hotter and hotter by self -compression and bound to grow hotter and 

 hotter so long as it remains a gas; and it is bound to remain a gas 

 until it falls below the critical temperature, which it can not do while 

 it continues to grow hotter; (2) a gravity that increases fourfold with 

 every halving of distance and that is bound to increase so long as con- 

 centration continues, and concentration must continue while the sub- 

 stance is a gas and the gravitative pressure increases. 



What is the logical outcome of this kind of logic and this sort of a 

 combination 'i A geologist begins to grow dizzy contemplating such 

 thermal possibilities. Why should not atoms, atomecules, and what- 

 ever else lies below, one after another, have their energies squeezed 

 out of them, and the outer regions be heated and lighted for an 

 unknowable period at their expense ? 



There was a time when the chemical theory of the sun's heat was 

 fairly satisfactory^ to the scientists of the day, but its inadequacy 

 appeared in time. There followed a period in which the meteoroidal 

 theory of the sun's origin was deemed adequate, but its defects soon 

 became apparent. There has followed the contractional theory, the 

 validity of which is perhaps not less questioned now than was the 

 validity of the chemical and meteoroidal hj'potheses in their day of 

 acceptance, but, judging from the past, it may easily appear in the 

 future that the Helmholtzian theory is inadequate in some measure 

 not unlike its predecessors. 



But assuming, as we are wont to do, that the limits of our present 

 knowledge are a definition of the facts, has the evolution of the sun 

 been worked out with such clefiniteness and precision as to give a deter- 

 minate and specific history of its thermal stages from beginning to end ? 

 It is one thing to tell us, on the basis of the contractional theory, that 

 the total amount of thermal energy originally potential in the S3^stem 

 is only equal to so man}- million times the present annual output, but 

 it is quite a difi'erent thing to give a specific statement of the actual 

 time occupied by the sun in the evolution and discharge of this amount 

 of heat and to define its successive stages. It is with this actual history 

 that we are specially concerned. The distribution of the computed 

 heat in time may ha^-e been such hypothetically as to shorten the period 

 of it.s expenditure not simply to twenty or twenty-five millions of years, 

 as indicated by Lord Kelvin, but to four or six millions of years, as 

 deduced by Ritter. ' On the other hand, the dealing out of this amount 



1 Astrophysical Journ.il, December, 1898; Journal of Geolosjv, p. 93, No. 1, Vol. VII, 

 1899. 



