WHAT WE KNOW ABOUT COMETS 539
its mass. Electric and radiation-pressxire repulsions will therefore act more eflSciently upon very small particles than upon large ones. A cube of water one centimeter on each edge would be drawn by the sun's gravita- tional action 10,000 times as strongly as the pressure of the sun's rays fall- ing upon that body would repel it. But a cube of water only 1/1000 of a mm. on each edge would be in equilibrium under the sun's gravitational attraction and the sun's light-pressure repulsion. A cube of water le^fl than 1/1000 mm. would actually be driven rapidly away from the sun. The equilibrium diameter for little spheres of water, according to Nichols and Hull, is .0015 mm. Now as light energy is traveling along with a speed of 186,000 miles a second, we should expect particles of matter considerably smaller than the equilibrium size to travel away from the sun with great and rapidly increasing speeds. These speeds would be the greater for particles smaller and smaller until a certain limit of size with reference to wave-length of light is reached, after which the light would be diffracted without transmitting so large a proportion of its repulsive energy to the particles. These limits of efficiency were determined by the lamented Schwarzschild. The re- sistance of cometary particles is evidently also a function of the specific gravity of the particles. The figures which we have quoted are for water, density 1. We can scarcely doubt that radiation pressure is an important force, perhaps the chief force, perhaps the only force respon- sible for the driving out of the materials of comets' taib. Particles of solid matter or gas molecules of three different classes of sizes might be responsible for the three main classes of comets' tails. More prob- ably materials of three different classes of density compose the three classes of tails. Bredichin called these three classes the hydrogen, the hydrocarbon and the iron tails. The atomic weights of these three substances give to their atoms or molecules about the right mobility, under equal pressure upon all, to explain the lags of the three classes of tails. Unfortunately it is far from certain that hydrogen exists in comets, and iron has been reported for only one comet.
The hoods or envelopes (Fig. 4) which form the outer strata of the heads of comets which come close to the sun are very interesting. It is the prevailing view that, when a comet approaches the sun, the solar heat falling upon that surface of the comet which faces the sun gen- erates or liberates the gases and vapors which have been contained in or between the more solid parts of the comet; and being liberated, in effect, under pressure, the materials at first travel toward the sun with considerable speed. The sun's repulsive force acts upon these jets and, overcoming the forward motion of the materials, it eventually turns them back along the tail. Those phenomena have been observed many times.
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