Page:Popular Science Monthly Volume 28.djvu/185

Rh filled with some substance having, as we have already seen, the properties of a solid. Now, although it is easier to conceive that all space is filled with some kind of substance than to conceive it to be empty, in order to account for universal gravitation, it is at least unexpected that this substance should turn out to be a solid, yet the polarization of light shows that a solid substance it must be, notwithstanding the fact that the planets rush through it without the smallest apparent resistance.

But even this anomaly is not utterly inconceivable, for many familiar substances have at one and the same time the properties both of a solid and a liquid—for example, pitch, rosin, and tar. We would all probably consider pitch as quite a brittle solid, yet it is at the same time a perfect liquid, as an incident that happened to Alvan Clark will illustrate. He once opened a new barrel of pitch, using a hatchet to crack off some for use in polishing lenses; after breaking off enough for his purpose, he laid the hatchet down on the pitch which nearly filled the barrel, and thought no more about it until some few weeks afterward, when the hatchet could not be found, although he distinctly remembered having left it lying on the opened barrel. He thought it stolen until about two years afterward, when the missing hatchet was discovered at the bottom of the pitch, having sunk into it, clear to the bottom, leaving no hole behind, just as a stone would sink in water, only of course much more slowly.

All who have worked with pitch know that it has the property of being a slowly moving liquid; and it is evident that this particular kind of substance at least is a solid to one kind of motion, such as the quick blow of a hatchet, but is a liquid to another kind of motion, such as the steady pressure of a hatchet slowly descending through it. That is, give it plenty of time to flow, and pitch is a perfect liquid; but hurry it, and it is a very brittle solid.

Now, this strange substance which fills all space seems to possess this peculiar double property in a vastly greater degree than does common pitch, for we find that to such a quick motion as a vibrating molecule it acts as a most rigid solid, but to the comparatively slow and steady motion of a planet it acts as an inconceivably thin liquid, allowing the planet to pass through with no apparent resistance.

This remarkable substance, which fills both intermolecular and interstellar space, is called the universal ether. Its properties are only beginning to be learned, and will not probably be well understood until such phenomena as gravitation, electricity, magnetism, and the peculiarities seen in the tails of comets, are satisfactorily explained. A statement, however, of a few of its observed properties is a necessary prelude to a complete understanding of the telescope.

The molecules of ponderable matter are supposed to be inclosed in the ether, just as a wooden ball could be incased in the center of a large block of jelly. The waves of light are supposed to be originated