408 NEBULOSITY OF COMETS. [SECT, xxxvi. 



ever, that the ethereal fluid would penetrate the nebulous 

 matter instead of compressing it. Sir John Herschel, on 

 the contrary, conjectures that it may be owing to the 

 alternate conversion of evaporable materials in the upper 

 regions of the transparent atmosphere of comets into the 

 states of visible cloud and invisible gas by the effects of 

 heat and cold; or that some of the external nebulous 

 envelopes may come into view when the comet arrives at a 

 darker part of the sky, which were overpowered by the 

 superior light of the sun while in his vicinity. The first of 

 these hypotheses he considers to be perfectly confirmed by 

 his observations on Halley's comet, made at the Cape of 

 Good Hope, after its return from the sun. He thinks that, 

 in all probability, the whole comet, except the densest part 

 of its nucleus, vanished, and was reduced to a transparent 

 and invisible state during its passage at its perihelion ; for 

 when it first came into view, after leaving the sun, it had 

 no tail, and its aspect was completely changed. A parabolic 

 envelope soon began to appear, and increased so much and 

 so rapidly that its augmentation was visible to the eye. 

 This increase continued till it became so large and so faint, 

 that at last it vanished entirely, leaving only the nucleus 

 and a tail, which it had again acquired, but which also 

 vanished; so that at last the nucleus alone remained. Not 

 only the tails, but the nebulous part of comets diminishes 

 every time they return to their perihelia; after frequent 

 returns they ought to lose it altogether, and present the 

 appearance of a fixed nucleus : this ought to happen sooner 

 to comets of short periods. M. de la Place supposes that 

 the comet of 1682 must be approaching rapidly to that 

 state. Should the substances be altogether, or even to a 

 great degree, evaporated, the comet would disappear for 

 ever. Possibly comets may have vanished from our view 

 sooner than they would otherwise have done from this 

 cause. 



If comets shine by borrowed light, they ought, in certain 



