THE BIRTH OF OUR WORLD 9 



But for some time it was supposed that one element was to be 

 excepted which appeared to exist only in the sun and had for 

 that reason been called helium. Helium, however, has now 

 been discovered on the earth as one of the products of the dis- 

 integration of radium, and, since the discovery of its origin, 

 it has played a considerable role in the speculations of 

 physicists. The study of the spectra of stars has not revealed 

 any special bodies. Those of the nebulae have given us onty two, 

 nebulium and archonium ; so we reach the conclusion that 

 our whole universe is made up of the same substances, which 

 is, so to speak, quite natural if atoms are merely ether animated 

 by certain vortical movements. 



It is still more natural that the substance of the different 

 planets should be identical, on the hypothesis that they have 

 come from the sun, as Buff on already believed, and as has 

 been accepted by all astronomers since Laplace. The origin 

 of these stars is not due to chance ; it occurred at definite 

 periods which seem to correspond to successive phases of the 

 contraction and cooling of the sun. During the period in which 

 they were formed the elements composing the sun were already 

 arranged in the order of their increasing density and in what 

 might be called that of their viscosity. The most distant 

 planets, the first in all probability to be formed, are very large 

 and very light, and since they remained in a molten state for 

 a very long time they themselves gave birth to a large number 

 of satellites, that is, to numerous moons. 1 These planets are 

 Neptune, Uranus, Saturn, and Jupiter. Then, suddenly, come 

 the denser and smaller planets, with only a small number of 

 satellites : Mars, the Earth, Venus, and Mercury. Between 

 these two groups and within the same orbit, revolve a great 

 number — almost a thousand — of small stars, the asteroids. 

 It may be conjectured that between Jupiter and Mars there 

 once was a planet containing so large a proportion of light 

 matter similar to that of the larger planets, or of heavy matter 

 similar to that of the planets analogous to the earth, that 

 since all these substances contracted unequally in cooling, the 

 planet broke up like a piece of glass of heterogeneous origin 

 in the fire ; and that these fragments were then scattered 

 along the whole length of its orbit. This hypothesis 



IV, 6. 



