W. A. Norton—Coggia’s Comet. 167 
from the region exposed to the normal incidence of the sun’s 
rays, it appears, then, altogether fails of application to Coggia’s 
. comet. It gives the single hollow paraboloidal tail without 
duplication, which is entirely at variance with the facts of ob- 
servation. 
Physical Theory of cometary phenomena.—Now, if there be, 
in fact, two systems of jets, eminating from opposite hemis- 
pheres of the nucleus, and passing over from one to the other, 
we can look for the origin of such a state of things only in a 
supposed magnetic condition of the nucleus, and in the hypoth- 
esis that the lines of initial discharge lie in the direction of the 
. lines of the magnetic force ; or in lines having a certain relation 
to these. This consideration brings us to the proper point of 
view for the presentation of the definite physical theory of com- 
nucleus. This envelope, consisting of a diamagnetic gas, is 
traversed by the ideal lines of magnetic force proceeding from 
the nucleus, which are also lines of electric conduction through 
the diamagnetic gas. The electricity set free by the ascending 
currents of the gas, by reason of the diminished gaseous pres- 
sure, is propagated along these lines; and the impulsive force 
of the electric currents detaches streams of successive mole- 
cules of the gas, in the direction of the lines of conduction. 
La Rive’s well-known experiment of transmitting electricity 
through an attenuated gas or vapor surrounding a magnet, 
showed that the lines of force in the magnetic field were also 
Operate on other molecules not thus detached, with an intensity 
‘sufficient to overcome their gravitation toward the nucleus. 
' In my mathematical discussion of Donati’s comet* I reached 
the result that the tail of the comet was made up of matter of 
which a portion was solicited by an effective solar repulsion, 
* This Journal, Il, vol. xxxii, No. $4, July, 1861. 
