# Page:Popular Science Monthly Volume 60.djvu/274

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POPULAR SCIENCE MONTHLY.

Comets' Tails.

The single new principle introduced by Arrhenius arises in connection with the problem of comets' tails. Astronomers have always felt that the phenomena exhibited by these strange objects could only be accounted for by making the sun the seat of a violent radial repulsive force, but were entirely at a loss to account for this repulsion. So long as light was supposed to consist of myriads of corpuscles discharged with a speed of 186,000 miles per second, it was easy, with Kepler, to regard the corpuscles as carrying with them in their rush the materials vaporized from the comet by the heat of the sun. But the establishment of the Wave-Theory of light put an end to this idea. Thus Newcomb says ('Popular Astronomy'): "If light were an emission of material particles, as Newton supposed it to be, this view would have some plausibility. But light is now conceived to consist of vibrations in an ethereal medium; and there is no known way in which they could exert any propelling force on matter!"

Now Arrhenius points out that according to the Electromagnetic Theory of light a ray of light does exert a pressure on any surface on which it impinges. Maxwell not only proved this in his original publication of the theory in 1873, but showed how to calculate its value. With the known constants of solar radiation he found that sunlight at the surface of the earth should exert a pressure of.592 X 10-10 grams on every square centimeter. This is too small a force to be detected, though it has been looked for.

But at the surface of the sun the pressure would mount up to 2.75 milligrams per sq. cm. On the other hand, a cubic centimeter of water, which weighs one gram at the surface of the earth, would weigh 27.47 grams at the surface of the sun, i.e., the attraction of the sun would draw it inwards with about 10,000 times the force with which the sun's light would tend to drive it away.

Very different is the case if, instead of a cubic centimeter, we consider a much smaller cube. The pressure on its base would fall off as the square of its edge, but the weight would diminish as the cube. There must come a point at which the pressure of the light would just balance the weight; and still smaller particles would be driven off with a force greater than their weight. They would behave, in fact, as if gravity had become negative.

For example, a cube of water measuring one-thousandth of a millimeter (10-4 cm.) in the edge would weigh 27.47${\displaystyle \times }$10-12 gms.; and the pressure of light on its base would be 2.75${\displaystyle \times }$10-3${\displaystyle \times }$10-8, ${\displaystyle =}$ 27.5 ${\displaystyle \times }$ 10-12 gms., i.e., slightly more than its weight.

In measuring wave-lengths of light physicists denote one-thousandth of a millimeter by the symbol μ. The critical value of the edge of a cube of water, i.e., the value for which its weight is exactly neutralized