ADDRESS. 



11 



metals and meteorites by a disruptive discharge, and carried forward into 

 the tube of observation by a more or less rapid current of air or other gas. 

 These experiments prove that metallic dast, however fine, suspended in a 

 gas will not act like gaseous matter in becoming luminous with its cha- 

 racteristic spectrum in an electric discharge, similar to that of the Aurora. 

 Professor Schuster has suggested that the principal Hue may be due to 

 some very light gas which is present in too small a pi-oportion to be 

 detected by chemical analysis or even by the spectroscope in the presence 

 of the other gases near the earth, but which at the height of the auroral 

 discharges is in a sufficiently greater relative proportion to give a spectrum. 

 Lemstrom, indeed, states that he saw this line in the silent discharge of 

 •a Holtz machine on a mountain in Lapland. The lines may not have 

 been obtained in our laboratories from the atmospheric gases, on account 

 of the difficulty of reproducing in tubes with sufficient nearness the 

 conditions under which the auroral discharges take place. 



In the spectra of comets the spectroscope has shown the presence of 

 •carbon presumably in combination with hydrogen, and also sometimes 

 with nitrogen ; and in the case of comets approaching very near the sun, 

 the lines of sodium, and other lines which have been supposed to belong 

 +0 iron. Though the researches of Professor H. A. Newton and of 

 Professor Schiaparelli leave no doubt of the close connection of comets 

 -with corresponding periodic meteor swarms, and therefore of the probable 

 identity of cometary matter with that of meteorites, with which the 

 spectroscopic evidence agrees, it would be perhaps unwise at present to 

 attempt to define too precisely the exact condition of the matter which 

 forms the nucleus of the comet. In any case the part of the light of 

 the comet which is not reflected solar light can scarcely be attributed 

 to a high temperature produced by the clashing of separate meteoric 

 stones set up within the nucleus by the sun's disturbing force. We must 

 look rather to disruptive electric discharges produced probably by pro- 

 cesses of evaporation due to increased solar heat, which would be amply 

 sufficient to set free portions of the occluded gases into the vacuum of 

 space. May it be that these discharges are assisted, and indeed possibly 

 increased, by the recently discovered action of the ultra-violet part of the 

 «un's light ? Hertz has shown that ultra-violet light can produce a dis- 

 charge from a negatively electrified piece of metal, while Hallwachs and 

 Righi have shown further that ultra-violet light can even charge posi- 

 tively an unelectrified piece of metal ; phenomena which Lenard and 

 Wolf associate with the disengagement from the metallic surfaces of very 

 minute particles. Similar actions on cometary matter, unscreened as it is 

 by an absorptive atmosphere, at least of any noticeable extent, may well 

 be powerful when a comet approaches the sun, and help to explain an 

 electrified condition of the evaporated matter which would possibly bring 

 it under the sun's repulsive action. We shall have to return to this 

 point in speaking of the solar corona. 



