488 



PHYSICAL CONSTITUTION OF COMETS. 



this theory is tainted by a fatal error. It proceeds upon the supposition that 

 the comet, in the one hand, is formed of an elastic gas or vapor ; and, on the 

 other, that it is impervious to the solar atmosphere through which it moves. / 

 To establish the theory, it would be necessary to suppose that the elastic fluid 

 composing the comet should be surrounded by a nappe or envelope as elastic as 

 the fluid composing the comet, and yet wholly impenetrable by the solar at- 

 mosphere. 



Several solutions of this phenomenon have been proposed by Sir John Her- 

 schel :* one is, that the comet consists of a cloud of particles, which either 

 have no mutual cohesion, or none capable of resisting their solar gravita- 

 tion ; that, therefore, these particles move round the sun as separate and inde- 

 pendent planets, each describing an ellipsis or parabola, as the case may be. 

 If this be admitted, it is demonstrable on geometrical principles, and, indeed, 

 it follows as a necessary consequence of the principle of gravitation, that the 

 particles thus independently moving, must converge as they approach the sun, 

 so as to occupy a more limited space, and to become condensed ; and that on 

 receding from the sun, they will again diverge and occupy increased dimen- 

 sions. 



Herschel insists on this the more, because he conceives it has the character 

 of a vera causa. The fact is, the hypothetical part of it consists, not in the 

 assumed effect of the gravitation of the particles of the comet, but in the as- 

 sumption that the mutual cohesion or mutual gravitation of these particles is a 

 quantity evanescent in comparison with their separate gravitation toward the 

 sun. This can scarcely be ranked as anything but a supposition assumed to 

 account for the phenomena. 



Another theory proposed by Sir John Herschel, which indeed is not al- 

 together incompatible with the simultaneous operation of the former cause, ia r 

 that the nebulous portion of the comet, or that portion which reflects the sun's 

 rays, is of the nature of a fog, or a collection of discrete particles of a vapor - 

 izable fluid floating in a transparent medium ; similar, for example, to the cloud 

 of vapor which appears at some distance from the spout of a boiling kettle. 

 Now, since these molecules, during the comet's approach to the sun, absorb its 

 rays and become heated, a portion of them will be constantly passing from the 

 liquid to the gaseous or invisible state. As this change must commence from 

 without, and must be propagated inward, the effect will be a diminution of the 

 comet's visible bulk. On the other hand, as it retreats from the sun, it will lose 

 by radiation the heat thus acquired ; which, in conformity with the general 

 analogy of radiant heat, will escape chiefly from the unevaporated or nebulous 

 mass within. The dimensions of this will therefore begin and continue to in- 

 crease by the precipitation immediately above it of fresh nebula ; just as we 

 see fogs in. cold and still nights forming on the surface of the earth, and grad- 

 ually extending upward as the heat near the surface is dissipated. The comet 

 would thus appear to enlarge rapidly in its visible dimensions, at the moment 

 that its real volume is in fact slowly shrinking by the general abstraction of 

 heat from the mass. 



" This process," says Sir John Herschel, " might go on in the entire absence 

 of any solid or fluid nucleus ; but supposing such a nucleus to exist, and to 

 have acquired a considerable increase of temperature in the vicinity of t'he sun, 

 evaporation from its surface would afford a constant and copious supply of va- 

 por, which, rising into its atmosphere, and condensing it at its exterior parts, 

 would tend yet more to dilate the visible limits of the nebula. Some such pro- 

 cess would naturally enough account for the appearances which have been 



* Memoirs Royal Astron. Soc., vol. vi., p. 104. 



