COMETS, THE SOLAR CORONA, AND THE AURORA. 47 



cause involving assumptions beyond conception or con- 

 trary to experience, must depend on the answers to the 

 following question : — Do we know, or can we conceive, 

 any physical state into which any substance which can be 

 conceived to occupy the space traversed by comets could 

 possibly be brought so as to make it present the appear- 

 ance exhibited by comets ? 



Now I think the answer must be in the affirmative, 

 and that we may leave out the terms conceive and con- 

 ceivable. For electricity is a well known state, and gases 

 are well known substances ; and when electricity under 

 certain conditions, as in Dr. Geissler's tubes, is made to 

 traverse exceedingly rare gas, the appearance produced is 

 similar to that of the comets' tails ; the rarer this gas is, 

 the more susceptible is it of such a state ; and, so far as we 

 know, there is no limit to the extent of gas that may be so 

 illuminated. Hence we may suppose the exciting cause 

 to be electricity, and the material on which it acts and 

 and which fills space to have the same properties as those 

 possessed by gas. What is more, we can conceive the sun 

 to be in such a condition as to produce that influence on 

 this electricity which should cause the tail to occupy the 

 direction it does ; for such an electric discharge will be 

 powerfully repelled by any body charged with similar elec- 

 tricity in its neighbourhood. 



The electricity would be discharged by the comets on 

 account of some influence which the sun may have on 

 them, such an influence being well within the limits of our 

 conception. 



The appearances of the comet in detail, such as the 

 emission of jets of light towards the sun and the form of 

 the illuminated envelope, are all such as would necessarily 

 accompany such an electrical discharge. 



In fact, if the possibility of such a discharge is admitted, 

 I believe it will explain all the phenomena of comets. 



