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SCIENCE 



[N. S. Vol. XL VI. No. 1199 



have been a fiery mass. This idea was first 

 suggested by Descartes in his "Prineipia 

 Philosophise, ' ' published in 1644. Accord- 

 ing to him, the earth, like every other celes- 

 tial body, was fonned by the aggregation 

 of primitive particles of matter which 

 have an inherent whirling motion. The 

 resultant sphere, after it has changed from 

 the gaseous to the molten condition, cools 

 and becomes covered by a solid crust. But 

 the central portion still retains its hot and 

 plastic condition, which is manifested by 

 the phenomena of mountain-building and 

 vulcanism. 



Leibnitz, thirty-six years after, in his 

 "Protogoea," which, however, was not 

 published until after his death, followed 

 an almost identical hypothesis, conceiving 

 the earth to have been built up of an aggre- 

 gation of whirling ultimate elements or 

 "monads" of matter. But while Descartes 

 looked upon the motion as being due to the 

 momentum supposed to be present in con- 

 stant amount in the universe, Leibnitz be- 

 lieved it to be due to the force which ac- 

 companied the separation of light from 

 darkness. 



Later this doctrine was carried a step 

 farther by the philosopher Kant, and 

 finally by Laplace in his theory so mod- 

 estly put forward, which has since become 

 so famous under the name of ' ' the Nebular 

 Hypothesis. ' ' 



Briefly stated this hypothesis predicates 

 the origin of our solar system in a great 

 fiery mass of incandescent vapor, similar 

 to the nebulae, which are among the most 

 wonderful objects revealed to us by the 

 telescope. The parent nebula of our sys- 

 tem must have extended far past the pres- 

 ent orbit of the outermost planet, Neptune, 

 then undiscovered. In order to fill this 

 space the matter available must have been 

 spread out extraordinarily thin; in fact, 

 the density would be one millionth of that 

 of the air we breathe. The whole was sub- 



ject to a rotary motion. As time passed, 

 heat was radiated into space, and, as the 

 tenuity was maintained by heat, the mass 

 became cooler and denser. Particles on 

 the circumference would thus steadily 

 move closer in to the center. Now the ve- 

 locity of any such particle would remain 

 unchanged, while the distance it would 

 have to travel in order to complete the jour- 

 ney around the center, would steadily grow 

 less. It follows that it would be whirling 

 around the axis at an ever-increasing 

 rate, and consequently, with an ever-in- 

 creasing tendency to fly off into space. At 

 the same time the pull of gravity, since the 

 particle is closer to the center, is constantly 

 growing greater. It is then subject to two 

 steadily increasing forces, one of which 

 tends to throw it off, the other to drag it 

 down. A time will come when these two 

 forces will just balance and the particle 

 will go up neither nor down, but remain re- 

 volving in an orbit. The total result of 

 this on all the particles of the outer zone 

 would be to leave them in the form of a 

 ring of gas. Similarly, the same process 

 would be followed in the case of another 

 zone, until the whole would resolve itself 

 into a central spherical nebula surrounded 

 by a series of rings. Each ring in turn 

 would soon break, and the gas of which it 

 was composed would come together in a re- 

 volving sphere, which might give rise to 

 other rings. The system is constantly 

 cooling, and the spheres of gas, finally 

 solidifying, give rise to the planets and 

 satellites. 



The simplicity and grandeur of this 

 theory fire the imagination. It is no 

 wonder that it took firm root. For sev- 

 eral generations it was received without 

 reservation. Gradually, however, serious 

 defects began to be seen. For instance, if 

 we calculate the rate of motion of the mole- 

 cules of such a system, the temperature 

 and rate of rotation of the whole being 



