BIELA'S COMET. 147 



forces, perhaps the expansion of gaseous substances. If 

 we knew that Biela's comet was a solid body, we might 

 easily suppose it to have been divided by some force 

 similar to volcanic agency. But most of the matter of 

 this body is of the rarest kind ; and it may be doubted 

 whether any part of it is in a solid state. 



Was this separation caused by a repulsive force emanat- 

 ing from the sun ? The phenomena exhibited by Halley's 

 comet at its return to the sun in 1835, require us to ad- 

 mit the existence of repulsive as well as attractive forces. 

 The effect of the sun's repulsion upon the atmosphere of 

 the comet, would be to distort it from the spherical form, 

 which it would assume under the attraction of the nucleus 

 alone to crowd the particles on the side next the sun 

 nearer to the nucleus, and to drive those on the opposite 

 side further from it causing an oval form, whose length, 

 as compared with its breadth, would be the greater, the 

 stronger the repulsive force is supposed ; and the repuls- 

 ive force may be conceived to become so great as to 

 drive the remoter particles beyond the influence of the 

 nucleus, and carry them off into space. It is necessary, 

 in the opinion of Sir John Herschel, to suppose that the 

 tail is attracted by the nucleus, otherwise they would at 

 once part company. The compound mass of the comet is 

 therefore urged toward the sun by the difference of the 

 total attractive and repulsive forces ; and so long as the 

 repulsive power is insufficient to separate them, they will 

 revolve together as one body, continually elongating 

 itself as it approaches the sun, and on the position of 



