Page:Astronomy for Everybody.djvu/311

posing the rays of light to be parallel, the work done by the pressure upon a surface moving through any length of the pencil is equal to the energy of the light contained in that length.

By the aid of this principle and a knowledge of the heat or energy contained in the rays of the sun, it is possible to calculate the pressure in question. It is found to be too slight to be detected by any ordinary mode of measurement. The great difficulty arises from the fact that, if the experiment is not tried in a vacuum, the pressure will be confused with that exerted by the surrounding air. A vacuum so nearly perfect that the slight residuum of air still contained within it shall not exert a force comparable with the light has not yet been attained. Our conclusion must therefore depend on observations made on minute particles contained in the celestial spaces; and we cannot ascend into these spaces to make the experiments, nor can we send matter up there to be experimented upon. All we can do is to observe matter already at hand. Here, then, is a wide gap which we cannot bridge over in practice.

The other element in the case is the discovery that particles smaller than atoms, called corpuscles or ions are thrown off with high velocity from intensely heated bodies. The sun being such a body, it follows that such ions must be shot out from it.

On Maxwell's theory, the explanation of a comet's tail is simple in the extreme. Being in the vacuum of celestial space, the matter of the comet evaporates on the side next to the sun, and, there being no pressure to hin-