38 



would not have been reduced 10 feet. However it may 

 be that all the material evapoi'ated 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 hydi'o-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. 



