39 



The jDeriod 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 liard 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 '7o to '55 the attraction of the 

 sun for ordinary matter. The matter repelled with '55, 

 forming the following edge, that mth '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 tv^o 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 a^s primary or secondaiy 

 tails. These I maintain can only be explained on the 



