258 THE NEBULAE HYPOTHESIS. 



one surface of the nebulous mass more than another 

 the conclusion must be that they will come to the cen- 

 tral body from various directions in space. This, too, 

 is exactly what happens. Unlike planets, whose orbits 

 approximate to one plane, comets have orbits that show no 

 relation to each other ; but cut. the 2>lane of the ecliptic at 

 all angles. 



In the third place, applying the reasoning already 

 used, these remotest flocculi of nebulous matter will, at 

 the outset, be deflected from their straight courses to the 

 common centre of gravity, not all on one side, but each 

 on such side as its form determines. And being left be- 

 hind before the rotation of the nebula is set ujd, they 

 will severally retain their different individual motions. 

 Hence, following the concentrating mass, they will event- 

 ually go round it on all sides ; and as often from right to 

 left as from left to right. Here again the inference per- 

 fectly corresponds with the facts. While all the planets 

 go round the sun from west to east, comets as often go 

 round the sun from east to west as from west to east. Out 

 of 210 comets known in 1855, 104 are direct, and 106 are 

 retrograde. This equality is what the law of probabilities 

 w^ould indicate. 



Then, in the fourth place, the physical constitution of 

 comets completely accords with the hypothesis. The abil- 

 ity of nebulous matter to concentrate into a concrete form, 

 depends on its mass. To bring its ultimate atoms into that 

 proximity requisite for chemical union — requisite, that is, 

 for the production of denser matter — their repulsion must 

 be overcome. The only force antagonistic to their repul- 

 sion, is their mutual gravitation. That their mutual gravi- 

 tation may generate a pressure and temperature of suffi- 

 cient intensity, there must be an enormous accumulation of 

 them ; and even then the approximation can slowly go on 

 only as fast as the evolved heat escapes. But where the 



