164 W. A. Norton—Coggia’s Comet. 
carbon-dioxide in the gaseous form—either continuously or in- 
termittently—and these may occur either simultaneously over 
large areas, or in limited streams or jets. A portion of the 
vapor evolved may be condensed into solid particles by the cold 
resulting from the rapid evaporation. In so far as the gas is in 
intimate physical association with the solid matter of the nu- 
cleus, it would seem that the heat of the sun would not at the 
perihelion distance of either Coggia’s Comet, the great comet of 
1861, Donati’s Comet, or, indeed, of any of the conspicuous 
comets with a few exceptions, be intense enough to occasion 
such copious evolutions of gaseous matter as have been actually 
observed. Evolutions of occluded gas may, however, consti- 
tute the chief phenomena, produced by the solar heat, in the 
cases of the inconspicuous comets of short period, which, in re- 
treating from the sun do not pass sn the limits of the 
planetary system. 
The conclusions that have now been reached apply strictl 
only to the comets that have been spectroscopically observed, 
but they may be regarded as probably applicable also to other 
comets that do not differ greatly from these in the circumstan- 
ces of their approach to the sun and recess from him, and in 
the physical phenomena they have presented. The great 
comets of 1848 and 1860, that approached very near the sun, 
may have given off, in great abundance, aqueous or other 
forms of yaporous matter, derived from the liquefaction and 
the earth, permeated by free electricity increasing in tension 
from the surface of the nucleus upward; and that whatever 
Physical — of the Comet.—From numerous published 
the comet, I select that in which its peculiar 
eatures are most conspicuous. The cut is a copy of a drawing 
