PRESIDENTIAL ADDRESS, B.A. 1871. 195 



luminous in its nucleus, on account of collisions 

 among its constituents, while its " tail " is merely 

 a portion of the less dense part of the train illu- 

 minated by sunlight, and visible or invisible to us 

 according to circumstances, not only of density, 

 degree of illumination, and nearness, but also of 

 tactic arrangement, as of a flock of birds or the 

 edge of a cloud of tobacco smoke ! What pro- 

 digious difficulties are to be explained, you may 

 judge from two or three sentences which I shall 

 read from Herschel's Astronomy, and from the 

 fact that even Schiaparelli seems still to believe 

 in the repulsion. " There is, beyond question, 

 " some profound secret and mystery of nature 

 " concerned in the phenomenon of their tails. 

 " Perhaps it is not too much to hope that future 

 " observation, borrowing every aid from rational 

 " speculation, grounded on the progress of physical 

 " science generally (especially those branches of 

 " it which relate to the ethereal or imponderable 

 " elements), may enable us ere long to penetrate 

 " this mystery, and to declare whether it is really 

 " matter in the ordinary acceptation of the term 



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