Since the gaseous particles receding from a comet's head can never again return to it, and since a comet is visible to us chiefly, and gen erally wholly, by the electrical illumination of this gaseous medium, it is clear that a comet must become less and less luminous after each return to the sun. But not only does it become less luminous on account of the loss of the material forming its tail, but the meteors com posing its head lilcewise become more and more widely distributed along the course of its orbit. This is in part due to the positive charge left by the tail, which neutralizes the gravitation — or pull of the various portions of the comet's head for one another— and therefore permits those portions of the meteoric swarm which are nearest to the sun to move at a higher speed than those portions which are more remote.
In some cases, like our August meteors, the distribution extends throughout the orbit, with but one luminous condensation, known as the comet of 1862 III. In other cases there are two or more condensations. Thus Biela's comet was seen to split into two parts. Four distinct comets were found following in the track of the great comet of 1668.
We must now. discuss the important ques tion of the origin of comets. It is known that the sun, with its attendant planets, is traversing space in the direction of the con stellation Lyra at a speed of about 11 miles per second. If ccmiets come from remote inter stellar space, or from the other stars, it is obvious that we should meet more comets com ing from Lyra than would overtake us coming from the opposite direction. Also that those comets we met would have a greater velocity relative to the sun than would those that might overtake us. Nothing of the sort is found, however—the distribution is uniform in all directions. There is but one conclusion to be drawn from this, and that is that all the comets that we have observed possess the same com mon speed as the sun, omitting relative motion, and travel in the same direction. In short, they are all of them parts of the same original gaseous mass from which the solar system condensed, being merely those portions which were originally left on the outside, before the great common rotation was established, and which now occasionally drop in toward the centre, and then fly back again to their original position on the outslcirts of the system. The normal path of every comet is therefore an ellipse. If the paths of some comets have be
come slightly hyperbolic through the action of some outside body, they can never again visit the sun. As we have just seen, a comet not belonging to our system would be most likely to approach us from the general direction of the constellation Lyra, with a high hyperbolic velocity. We are acquainted with the orbits of about 400 comets, but no such body is cer tainly found among them.
From this point of view it is interesting to determine of what chemical elements comets are composed. To this the spectroscope gives a ready answer, and shows that by far the most common elements are hydrogen and carbon in chemical combination. This answer, however, is only a partial one, because most comets do not approadi sufficiently near to the sun to enable their more refractory elements to be volatilized. The bright small comet of 1882 I (Wells') approached so close to the solar sur face that the distance at perihelion amounted to only about 5,000,000 miles. Some of its metallic constituents were accordingly vapor ized, the most prominent of them being sodium. The great comet of 1882 III approached within 300,000 miles of thc sun's surface. This was so near that not only the sodium but even some of the iron lines appeared in its spectrum. This is what we should have expected, judging by the chemical composition of most of the meteors that reach the earth's surface. We thus see that incidentally comets are able to give us information regarding the temperature that bodies would attain at different distances from die sun. When the metallic lines are present in the spectra of these comets the carbon bands vanish. This is precisely the effect that is produced in our laboratories whem the cause of the illumination is an electric cur rent. The current by preference selects the molecules which are the best conductors. If the illumination of the comet were due merely to heat, there is no reason why the carbon bands should have disappeared.
Few persons now living remember distinctly the great comet of 1843. It appeared suddenly in the northern hemisphere about the middle of March, and was not long visible. It was notable for the great length of its tail and its small perihelion distance,— the smallest on record. This amounted to only 511,000 miles. Its centre therefore came within 78,000, and the nearer side of its head within 32,000, miles of the sun's surface.