Page:Popular Science Monthly Volume 11.djvu/245

Rh To conclude, the experiment I have just recounted shows that excitation of a sensor nerve at any point of its length is propagated in two directions, both centrifugally and centripetally. The same is doubtless true of a motor nerve. Consequently, it is highly probable that, as Vulpian has held, the nerves are simple conductors, differentiated only by their functions, which latter depend upon the kind of apparatus at their two extremities: for motor nerves there is a motor nerve-cell and muscular fibre, and for sensor nerves a receiving nerve-cell and an impressionable termination.—La Nature.



OT many years ago the planets Jupiter and Saturn were regarded as solid bodies, their small relative density being supposed to be due either to the peculiar arrangement of the materials which compose them, or to the small specific gravity of the materials themselves. Within a few years, however, the nebular hypothesis has gained so strong a hold on the minds of astronomers, that the larger planets are regarded in a very different light as respects their physical condition from what they were a few years ago. It is now thought that at least Jupiter and Saturn, and probably Uranus and Neptune, have not yet cooled down from their heated and nebulous condition sufficiently to be habitable globes, and that, owing to their relatively high temperature, they have very extensive atmospheres.

It is quite possible for us to guess at the extent and density of their atmospheres, but, unless we subject our guesses to calculation, it cannot be said that they are of much use to us, since the planets are too far from us to allow direct observations to settle any points in dispute. Let us, therefore, assume Jupiter and Saturn to be globes of much higher temperature than the earth's, and that this temperature varies from the surface upward inversely as the square of the distance from the centre of the planet. Let us further assume that gases there follow the same law of expansion as terrestrial gases, and that this law holds for all temperatures above 32° Fahr. With these assumptions, it is possible for us to subject our guesses to calculation, and thus learn how far they are admissible.

As respects the telescopic appearance of those planets, it is plain that we have but little evidence of an atmosphere outside the highest cloud-layers. That their atmospheres extend above the highest clouds is perhaps certain; but if we attempt to explain the physical 