39 
The period of greatest display is not reached till after the 
comet has passed its perihelion, and the tail is visible for 
much longer after this than it was before it. Now, if we 
suppose the comet to be made up of hard and volatile 
substances, owing to the heat absorbed by the hard sub- 
stances the evaporation would lag somewhat behind the 
position of the comet, and consequently be greatest after 
it had passed its perihelion distance, just as a thick retort 
will continue to boil after the lamp has been removed. 
Hence we see that if the evaporation causes electrical 
separation in the comet, this will be at its maximum just 
when the display is observed to be at a maximum. 
This communication is not intended as an alteration of 
the views which I expressed in a former communication, but 
as an extension of those views, for I formerly advanced no 
hypothesis as to the possible cause of the electricity. Also 
with regard to the formation of tails, I wish to add somewhat 
to my former remarks. Professor Norton has shown that 
the primary tail of Donati’s comet might have been formed 
by matter emitted by the comet and repelled by the sun 
with a force equal to from '75 to '55 the attraction of the 
sun for ordinary matter. The matter repelled with '55, 
forming the following edge, that with '75, the leading edge 
of this tail. Professor Norton suggests that these forces may 
be electrical or magnetic. 
Accepting Professor Norton’s calculations as correct, it is 
certain that if for some cause or other the sun repelled 
negative electricity, and there were two streams of electrified 
matter leaving the comet, charged in the ratio of '75 to '55, 
these would be repelled in the ratio he wants ; at the same 
time I do not think he has sufficiently taken into account the 
repulsion one stream would have on the other. 
Professor Norton does not suggest an explanation of the 
straight tails seen with most comets as primary or secondary 
tails. These I maintain can only be explained on the 
