12 



SCIENCE. 



[N. S. Vol. XIV. No. 340. 



system was evolved ia some way from a 

 primitive nebula, we may say with certainty 

 that it did not follow the orderly course 

 marked out for it by Laplace. The subse- 

 quent discovery of the great principle of 

 the conservation of energy dispensed with 

 the original hypothesis which started with 

 ^he mass in a, heated condition, at the 

 ^isame time that it associated with it the im- 

 portant question as to the supply of heat and 

 other forms of energy which are constantly 

 being poured out with such prodigality by 

 the sun. If we suppose the matter com- 

 posing our system to have been at one time 

 a nebulous mass, iilling the present orbit of 

 Neptune, the temperature may then have 

 corresponded to the absolute zero so far as 

 our purposes are concerned. The process 

 of shrinking and condensation to the pres- 

 ent condition would have evolved an amount 

 of heat quite equal to that which the prob- 

 lem calls for, but, unless a constant supply 

 is furnished from some source, the present 

 process of radiation will soon come to an 

 eod. The explanation of this supply which 

 is generally accepted was first announced 

 by Helmholtz in 1854, He ascribed it to 

 the shrinkage of the sun now going on. It 

 is capable of mathematical proof that a 

 body consisting of matter in the form of a 

 gas, which is the case with the sun, by the 

 process of condensation due to the pressure 

 produced by its own attraction, will con- 

 stantly grow hotter so long as it remains 

 a gas. This operation must end when 

 a considerable portion of its matter is re- 

 duced to a liquid or solid form. The sys- 

 tem, then, had a beginning, and as a con- 

 sequence it must come to an end. Or 

 more properly speaking the present con- 

 dition of things can not last forever. 

 Thus Helmholtz concludes that if the 

 intensity of radiation has been uniform 

 from the beginning, the present order can- 

 not have existed longer than 22,000.000 

 years. Others make the period less. Look- 



ing into the' future, at the end of 5,000,000 

 years, the sun will have contracted to half 

 its present volume, and at the end of an- 

 other 5,000,000 years it will be mainly if 

 not entirely solid, and m'ust have ceased 

 to be self-luminous much earlier. An in- 

 teresting corollary to this subject is the 

 principle of tidal evolution developed by 

 Mr, G, H, Darwin, Supposing the moon to 

 have been separated from the earth by some 

 process at a time when the matter compos- 

 ing them was in a liquid condition, each body 

 would produce enormous tides in the other. 

 Consider those produced on the earth by 

 the action of the moon : the effect would be 

 on the one hand to retard the earth in its 

 rate of rotation, and on the other to drive 

 the moon farther from the earth. Without 

 going further into detail, we may say that 

 Mr. Darwin finds that if no other causes 

 were at work not less than 50,000,000 years 

 would be required for the evolution of the 

 system of the earth and moon as it now 

 exists. 



It was not far from the beginning of 

 the century that Herschel attempted the 

 solution of the greatest of all problems — 

 that of the structure of the universe. The 

 problem proved, as may be supposed, quite 

 impossible of solution by methods then avail- 

 able. Much has been learned during the 

 century which was unknown to Herschel, 

 but we seem to be as far as ever from the 

 final solution. Instead of an orderly dis- 

 tribution of stars, clusters and systems, 

 we' find all apparently intermingled with 

 vast cosmic clouds and huge dark bodies, 

 possibly burned-out suns. For anything 

 we know these latter may be as numerous 

 or more so than the brilliant ones. Will 

 the labor of another century bring order 

 out of this seeming confusion or will it onlj'- 

 disclose still greater complexities unknown 

 to us ? Time alone can tell, 



c, l, doolittle. 

 Univeesity of Pennsylvania. 



