# Page:Popular Science Monthly Volume 79.djvu/258

254 THE POPULAR SCIENCE MONTHLY

where 10 -27 is the mass of a negative electron and 6.6 X 10" 8 is the gravitational constant in the c.g.s. system. Comparing the two results, we see that the former is 10 42 times the latter.

In astronomical bodies gravitation is the predominant force. An idea of its magnitude can be gained by calculating the attraction between the earth and the moon, which are small bodies astronomically speaking. The earth's mass is about 6 times 10 21 tons, which is 80 times the moon's mass, and the distance between the two is about sixty times the earth's radius; hence the attraction = 6 X 10 21 • 1/80 • l/60 2 = 2 X 10 18 tons of force. To hold this system while it rotates about a common center would require about five million-million steel bars each one foot square and of tensile strength of thirty tons per square inch. Knowing the distance between the earth and the sun (23,000 times the earth's radius) and that the sun is about 330,000 times as massive as the earth, in like manner we can show that the force between the earth and the sun is greater than that of the earth and the moon. What must it be for double stars ! Surely the origin of such gigantic forces ought to be worth careful study.

When Priestley and later Coulomb enunciated the law of electrical attraction (inversely as the square of the distance) they ignored the intervening medium. It was shown by Henry Cavendish, although not published, and discovered independently by Faraday that the electrical attraction depends upon the nature of the medium. If we take a piece of glass having a specific inductive capacity of six and separate two charges by this glass the force between them is only one sixth of what it is when they are separated by the same thickness of air. Strange to say gravitation is not affected by the intervening medium. This may be due to the gossamery nature of matter; that is, that the size of the molecules is very small compared with the distance between them.

So far as we are aware, chemical action, temperature and change of state are without effect on gravitation. No one has succeeded in demonstrating that it takes time for its propagation. If it is propa- gated in time the rapidity far exceeds that of light.

If we have two bodies electrically charged in a field and introduce a third body there is a redistribution. There is nothing analogous to this in gravitation, for the introduction of a third body in no way lessens the attraction between the other two. The earth's attraction is the same for me whether alone or in a crowd.

Thus when we compare gravitation with other phenomena about which at least we know a little, so great are the dissimilarities that it seems almost to fall outside the bounds of the physical realm.

Having briefly touched the discovery, law, magnitude, characteristics and peculiarities, we are ready to review the attempted explanations of the mechanism.

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