402 ANNUAL OF SCIENTIFIC DISCOVERY. 



expense of the tail, may thus become enlarged and finally consolidated, as 

 shown in the compact masses of the meteorites." " Thus is the comet the 

 building material of the meteorite; but a meteorite is a small planet, ttie 

 destination of which is to unite with a larger planet, to advance by one step 

 the world's process of increase. Thus do we proceed from atoms of matter, 

 isolated in the universe, to smallest crystals, to the cometary tail, to the 

 nucleus of the comet, to the meteorite, and to the planet upon which we 

 walk." 



ON THE CONDITION OF THE NUCLEUS OF A COMET, AND THE 

 EVOLUTION FROM IT OF NEBULOUS MATTER. 



The following views respecting the constitution of comets are given by 

 Professor W. A. Norton, in a paper published in Silliman's Journal, Vol. 

 xx vi., No. 79: 



In the first place, I conceive the telescopic nucleus of a large comet to consist 

 of an atmosphere of aqueous vapor, or of a vaporous and gaseous atmos- 

 phere combined, condensed upon an inner nucleus more or less covered with 

 water, or water partly in the condition of ice. In the case of the telescopic 

 comets this central mass is probably altogether wanting. The vaporous 

 atmosphere of the nucleus experiences variations of electric excitement 

 under the influence of the sun, after the same manner that the earth's 

 atmosphere is affected by the sun. That an electric influence is directly 

 exercised by the sun upon the upper regions of the earth's atmosphere, or 

 the photosphere of the earth, appears to me to have been fully established. 

 When repeated electric discharges take place in the higher and rarefied 

 regions of the atmosphere of the comet, or of that of the earth, they must 

 have the etfect, according to the results of the recent experiments of M. 

 Pliicker, to arrange the vaporous matter in columnar masses, coinciding in 

 direction with the lines of magnetic force. We thus have auroral columns 

 in the comet's as in the earth's atmosphere. At the magnetic poles of the 

 nucleus these would have a vertical position; and from these points would 

 gradually decline from this position, until at the equator they would lie 

 parallel to the surface. Now, as a comet recedes from the sun its tempera- 

 ture falls, the suspended aqueous vapor begins to condense at certain depths 

 in its atmosphere; the electricity thus set free flows in a scries of electric 

 discharges, which follow the course of the auroral columns as soon as they ai'e 

 established. Condensations extending through a considerable vertical depth 

 in the upper atmosphere would also be attended with electric discharges 

 from the one elevation to the other. It is these electric discharges along 

 these auroral columns that, as I conceive, disengage the particles of aqueous 

 vapor, or nebulous matter so called, and impel them off with a certain veloc- 

 ity. The same discharges bring the expelled particles into a condition to be 

 repelled by the nucleus. How this result may be produced, cannot here be 

 adequately explained. As the temperature of the receding comet continues 

 to fall, the pi'ocess of condensation, and consequent evolution of aqueous 

 vapor, goes on, and the visible nucleus increases in size. It would seem, 

 from the observations of Mr. Bond on Donati's comet, that large masses 

 appeared to be disengaged at certain intervals. These phenomena may have 

 arisen from the occasional suspension of the electric discharges taking place 

 in the upper atmosphere. This would produce the appearance of the detach- 

 ment and expulsion from the surface of the nucleus of a ring of nebulous 



