A. MATHEMATICS AND ASTRONOMY. 23 



laminated by means of the spark of an induction apparatus ; 

 the glass bell-jar in which the vacuum is to be made is trav- 

 ersed by the two wire conductors of the apparatus, the one 

 vertical and the other horizontal, and the induction spark it- 

 self appears in the form of feebly luminous rays, whose col- 

 ored stratifications surround the horizontal conductor with 

 a luminous sheath of a well-marked blue color. The hori- 

 zontal wire having been made of a thin plate of platinum, 

 an independent electric current is passed through it, so as 

 to render it red-hot, and immediately the blue-colored sheath 

 of rarefied air is repelled from the red-hot platinum plate. 

 After having made all possible variations of this experiment, 

 he concludes that it demonstrates a repulsion between the 

 heated platinum and air. 12 A, X., 289. 



THE CONSTITUTION OF COMETS. 



Mr. Lockyer briefly reviews the state of our knowledge as 

 to the spectroscopic observations of comets. In general, ob- 

 servers seem to agree that these bodies consist, in part at 

 least, of not very dense incandescent vapor, while in some 

 cases very dense or possibly very complex vapors, or even 

 glowing solid substances, seem to have been present. Hug- 

 gins first suggested the idea that the rarer cometary vapors 

 might be composed of nitrogen, but subsequently suggested 

 the theory that a comet is composed of carbon, and that a 

 temperature prevails high enough to volatilize a portion of 

 this substance, ffivinsr rise to the three bands coinciding with 



7 CD CD O 



those of olefiant gas. Mr. Vogel has, however, shown that 

 this is a very questionable theory, and that we are only justi- 

 fied in concluding that a portion of the light emitted by the 

 comet is its own light, and very probably comes from glow- 

 ing gas. Mr. Lockyer, moreover, found the nucleus of Cog- 

 gia's comet deficient in blue rays, whence its temperature 

 must have been low, which conclusion is further justified by 

 the fact that cometary light gives channeled space spectra, 

 which latter are peculiar to low temperatures. 12 A, X., 180. 



THE FORMATION. OF THE TAILS OF COMETS. 



The distinguished Italian astronomer, Schiaparelli, has com- 

 municated to the journal of the Italian Spectroscopic Society 

 some studies upon the nature of the repulsive force which 



