several instances two or more comets are known to be following practically the same path, and it was but a step from this to see that the head of a comet was only a concentrated swarm of meteors. It is only within the last few years, however, that we have begun to understand the nature of a comet's tail. That it is matter in a state of extreme tenuity is obvious, since, when millions of miles of it is interposed between us and the faintest stars, they are still visible and practically undimmed. What has puzzled astronomers since the time of Newton, however, is the fact that while all other bodies in the sidereal universe, as far as we are aware, obey the law of gravitation, comets' tails are clearly subject to some strong repulsive force, which drives the matter com posing them away from the sun with enor mously high velocities. The tail, therefore, always lies outside of the comet's orbit, being somewhat behind the comet when the latter is approaching the sun, and somewhat in ad vance of it when the comet is receding.
That the tail is gaseous is proved conclu sively by the spectroscope, also that it is in an extremely rarified condition; but why the ex tremely small particles which constitute a gas should act in any way differently under the law of gravitation from the larger meteoric masses which constitute the head of the comet is not at first sight obvious. If, however, we consider the matter carefully, we shall see that if all objects are attracted to the sun in proportion to their mass, and repelled from it in propor tion to their surface, then the attraction will vary as the cube of the diameter of the objects and the repulsion as the square. For objects of ordinary size the repulsion is so slight that we cannot detect it; but if the object becomes gradually smaller, the attraction of the mass will diminish much more rapidly than the re pulsion on the surface, and a time will come when they will be equal, and if the particle be comes still smaller, the repulsion must exceed the attractive force. It therefore becomes evident that if such a repulsive force existed, and if the gaseous molecules were sufficiently small, we should have an effect exactly like that which we observe.
It has been shown by J. J. Thomson ceeding of the Royal Society,) LVIII No. 350) that if hydrogen gas is electrified positively, the green line in its spectrum will be brighter than the red, and, on the other hand, if negatively charged, the red will be brighter than the green. In the case of the sun the red is the brighter line, so that we may infer that the solar surface is charged negatively.
It is a,well-lcnown fact that if the ultra violet rays of the spectrum be allowed to fall upon a metallic body little corpuscles or elec trons • leave the atoms forming the metallic body and fly away from them with enormous velocity. These corpuscles either carry a nega tive charge of electricity, or what is perhaps more probable, constitute the negative electric ity itself. The atoms lacking these corpuscles are as we usually express it positively charged.
Surrounding the meteor swarm and with it forming the comet's head is a mass of gas. When a flying corpuscle comes in contact with a molecule of this gas the two unite, the mole cule becomes negatively charged, and both be ing repelled by the negatively charged sun, recede from it. The subject will be found treated in more detail by It A. Fessenden, Astrophysical Journal, III, 36. He there com putes the potential of the sun's surface at about 15,000 volts. According to Arrhenius and Thomson the corpuscles which constantly deliver a negative charge to the earth, and would therefore also deliver it to a comet, proceed directly from the sun. The tail prob ably receives a charge from both these sources, but the latter would seem to be much the more effective of the two.
The question may now naturally be asked, since there is an abundance of extremely rarefied gas at an altitude of a few hundred miles above the earth's surface, and since we too are exposed to the flying corpuscles, why is not the earth itself provided with a comet like tail? In answer we reply that at certain times it is. of Harvard Observatory,' XXXII, 288. Our great auroras are indeed nothing else than the appearance of a small cometary tail. These great auroras, which exist at an altitude of several hundred miles, and envelope the whole earth, must not be con founded with the small but intensely brilliant local auroras always existing at low altitudes in the polar regions.
The reason that these great auroras only reach an altitude of a few hundred miles, in stead of several millions, like the tail of a comet, is on account of the great mass of the earth, which does not permit the electrified gases to escape from it. The great auroras exhibit two notable characteristics. They ap pear only when the electromagnetic condition of the sun is greatly perturbed, and they are most conspicuous at those times when the earth is approaching or receding most rapidly from it; that is, at the end of March and September.