Such, briefly stated, is the nebular theory. Among the facts 
seeming to establish its probability are: the revolution around 
the sun in the same direction of all the planets, which is the direc¬ 
tion, also, of the sun’s rotation upon its own axis, and of that 
of all the planets upon their axes—with the exception of Uranus 
and possible Neptune—and of the satellites about their primaries. 
Further, the orbits of all the planets are in very nearly the same 
plane, and this plane coincides with that of the sun’s equator. A 
striking illustration of the manner in which our solar system is 
supposed to have arisen from the original nebulous mass has been 
given in an experiment devised by M. Plateau. A quantity of 
oil is poured into a mixture of alcohol and water: the oil sinks 
to a certain depth, the lower strata of the mixture being heavier, 
and the upper strata lighter, than the oil, which, in this way, floats 
free, to a certain extent, from terrestrial gravitation, its particles 
seeming to be influenced merelv by their own mutual attractions, 
b\ reason of which the oil-mass assumes a spherical form. It is 
now gently caused to rotate about its centre, which causes a fudg¬ 
ing of its equator and a flattening of its poles—corresponding to 
the form of our sun and the planets. The rotation now being 
accelerated, the equatorial portion of the globe is gradually thrown 
off as a ring, which then continues to revolve about the parent- 
globe. The rotation of the whole mass continuing, the oil-ring 
breaks into fragments, and these revolve not only about the central 
mass, but, also, each upon its own axis, like the planets. 
During these transformations assumed by the nebular theory, 
the many masses, through their constant loss of heat, gradually 
passed from the gaseous into the liquid or molten condition, and 
then, first at the surface, solidification began. Our sun is still in the 
incandescent gaseous condition, losing its heat very slowly, partly 
because so large, and partly through the fresh generation of heat 
within it in a way as yet not understood. Our moon has passed 
through the gaseous and molten stages, ami is now a burned out 
solid, having been so small as to lose its heat with comparative 
rapidity. Our earth holds a middle position in this respect, having 
finished, it would seem, the gaseous stage, and being now for the 
most part molten, with a thin but ever thickening crust of solid 
matter. 
