THE AGE OF THE SUN. 631 



Dr. Croll, the distinguished Scotch geologist, has recently offered a theory 

 which is not altogether new, but admits of some novel arguments, and which 

 may serve all parties to the dispute. Beginning with a revievv of the differ- 

 ent theories as to the sun's heat, he rejects the combustio^a theory as totally 

 inadequate, since if the sun were all a burning coal, it would not last over 

 5,000 years; the chemical theory does not prolong the duration sufficiently; 

 the meteoric theory will not serve; the only remaining theory is the grav- 

 itation or condensation theory. This supposes that the materials of the 

 solar system were originally a nebula, extending through a space many 

 times greater than the orbit of Neptune. The falling together, the conden- 

 sation of this amount of matter, it can be mathematically shown, would 

 supply enough heat to keep the sun at its present temperature for 20,000,000 

 years. Unfortunately, that period is not sufficient for the geologist. He 

 demands at least 100,000,000 years for the changes of the earth's surface and 

 would prefer twice that length of time. The arguments of the geologist are 

 almost unanswerable ; those of the biologist who believe in evolution tend 

 to the same point so far as they go. Prof. Croll says that there is a way 

 out of this difficulty, by supposing that the nebula was not cold but hot. If 

 you suppose it hot enough to start with, you will have heat enough' to carry 

 you through. Obviously it is just as easy to suppose a hot nebula as a cold 

 one. But Prof. Croll proposes to provide for this original heat. If the solar 

 system had originally consisted of two masses, each of half the density of 

 the whole, at some immeasurable distance apart, and they fell foul of each 

 other owing to their mutual gravitation, they would strike with a speed of 

 L'7-4 miles per second. If their motion was stopped by the concussion, an 

 amount of heat would be developed sufficient to convert the whole into a 

 nebula that would take 50,000,000 years to cool. This is decidedly an im- 

 provement on the cold nebula. But this supposes that the component halves 

 before they started on their vvay to a collision; had no motion. Let us sup- 

 pose that they were moving beforehand at the rate of 202 miles per second, 

 and that this speed was added to what they got by gravitating toward each 

 other ; then we get, when they struck, a nebula extending beyond Neptune, 

 and with heat enough for a sun of 100,000,000 years' of duration. If you 

 insist on 200,000,000, you must give the original masses a speed of 676 miles 

 per second, beforehand. It will be objected that no such motion has been 

 observed in space.. Even the planets do not make such fast time ; the earth, 

 for instance, going only a thousand miles in a minute. The fixed stars, 

 whose motion has been ascertained, travel very much slower. But Prof. 

 Croll says the fixed stars are those that have gone through the collision 

 process,, and have lost their motion. The new hypothesis goes behind the 

 ordinary nebular theory in point of time, giving an explanation for the for- 

 mation of the nebulae. But it ^presupposes that there may be vast, cold, in- 

 visible masses of matter rushing through space with such velocity that their 

 mere touch would convert our globe into red hot gases and distribute it 

 through infinite space. The conception is not incompatible with the sudden 



