378 LIGHT OF COMETS. SECT. XXXV. 



which increases most rapidly in density towards the surface of 

 the sim, and forms an extensive atmosphere around him. It 

 did not occur to M. Valz, however, 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 per- 

 fectly 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 head, vanished, and was reduced to a trans- 

 parent 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, bnt 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. 



The comet discovered at Florence by Signore Donati, on the 

 2nd of June, 1858, was one of the most beautiful that has 

 been seen from our planet for many years, whether for the 

 brightness of the nucleus, or the length and graceful form of the 

 coma ; when first discovered it was near the star X in the con- 

 stellation of the Lion, being then at a distance of 288,000,000 



