38 
would not have been reduced 10 feet. However it may- 
be that all the material evaporated is not lost. For, 
from the way in which comets approach and recede 
from the sun, it is probable that part of their orbit lies 
without and part within the range of permanent evapo- 
ration. Hence during part of their motion, when they are 
distant from the sun, condensation will be going on if there 
is anything to condense. This agrees well with the observed 
fact that a periodic comet makes less and less display each 
revolution. There the heat acts on the surface of the comet 
so that the less volatile substances would form a skin over 
the softer ones, through which the heat would have to 
pass, and through which the steam would have to force 
its way in jets. 
Now such jets as these would act the same part as the 
jets in Armstrong’s hydro-electrical machine, and the vapour 
which emerged would be charged with either positive or 
negative electricity as the case might be, the solid being 
charged with electricity of the opposite kind. 
The vapour as it formed an atmosphere round the nucleus 
would then discharge some of the electricity back. This 
would cause those portions which were nearest the nucleus to 
be bright (self-luminous), brighter than the more distant. 
Although the variations in temperature would be slight, 
yet as the atmosphere moved outwards from the nucleus 
there would be expansion, and consequently condensation ; 
hence the outside of the coma might be illuminated by the 
direct rays of the sun, or we might have several bands of 
condensed vapour so illuminated, as suggested by Sir J. 
Herschel. On the other hand, I think this illumination 
may be due at least in part to the electric action between 
the matter of the comet and matter previously in space. 
This point will probably be settled by Mr. Huggins when 
the next large comet makes its appearance. 
