1889.] of the various Species of Heavenly Bodies. 



167 



It will be seen that in each of these cases the observations were 

 made when the comets were at a considerable distance from perihelion, 

 when the temperature would not be very high, although higher than 

 that which gives Mg 500. 



Carbon Radiation. 



When a comet gets nearer the sun there is a change in its spectrum 

 similar to that observed in the experimental tube at the second stage 

 of heat. Not only does the magnesium radiation change, as we have 

 seen, but the spectrum of carbon, produced from some compound of 

 carbon or another, in nineteen cases out of twenty when the comet 

 gets nearer the sun, and near enough to the earth to be satisfactorily 

 observed, becomes most prominent. 



Under these conditions, under which comets generally lend them- 

 selves best to spectroscopic study, the spectrum consists chiefly there- 

 fore of the flutings of hot carbon. In the majority of cases the 

 spectrum of a comet has not been recorded until it has arrived at 

 this stage of temperature. 



The three chief flutings of hot carbon have their least refrangible 

 maxima at approximately 517, 564, and 474. The accompanying 

 table indicates some of the comets in which they have been observed. 

 The variations in the position of the citron band will be again re- 

 ferred to. 



It is necessary to state that the maxim am luminosity of the blue 

 band, under some conditions, is at about 468. As I have so often 

 had occasion to refer to this, I here reproduce (fig. 2) one of the 



FIG. 2. Spectra of Alcohol at different Pressures. 

 1. Highest pressure. 2. Lower pressure. 3. Lowest pressure. 



