ON COMETS. 141 



affected one way or the other. The attraction on one 

 of its sides would precisely equal the repulsion on the 

 other. The separation of one portion of the matter of 

 a comet from the other by the action of the sun, which 

 we see, unmistakably, operated at and near the peri- 

 helion passage (a separation which the late Sir William 

 Herschel certainly had in mind, though perhaps some- 

 what indistinctly, when he spoke of a comet visiting our 

 system for the first time as consisting of "unperiheli- 

 oned " matter in contradistinction to those which he con- 

 sidered to have lost their tails by the effect of repeated 

 appulses, and to consist mainly of pej-ihelioned matter) 

 this separation I can only conceive, as I have ven- 

 tured to express it above, as an analysis of the mate- 

 rials : analogous to tliat analysis or rather disunion by 

 the action of heat which St Clair Deville has lately shown 

 to take place between the constituents of water at high 

 temperatures. In this latter case the chemical affinity is so 

 weakened that the mere difference of difficulty in travers- 

 ing an earthenware tube suffices to set them free of one 

 another. How much more so, then, were the one con- 

 stituent of a chemical compound subject to a powerful 

 repulsion from a centre which should attract the other, 

 and with it by far the larger mass of the total comet. 

 !Might not, under such circumstances, the mere ordinary 

 action of the sun's heat sufficiently weaken their bond of 

 union : and might not die residual mass, losing at every 

 return to the perihelion more and more of its levitating 

 constituents, at length settle down into a quiet, sober, 

 unexcitable denizen of our system % 



