350 Mr Milne^s Prize Essay on Comets. 



heat is not so great as Newton was inclined to estimate, it may be sup^ 

 posed that tlie variations of temperature to which a comet is subjected, 

 are yet much too considerable for the existence and abode of beings, 

 possessing constitutions at all analogous to those upon the Earth. But 

 an application of the laws of chemical science to this subject, demon- 

 strates that these extremes of heat and cold are by no means so exces- 

 sive, as the mere alterations in the comet's distance from the sun might 

 perhaps lead us to imagine. 



" In the first place, it is well known, that, in the heating of bodies, 

 when the compression to which they are subjected remains the same, 

 there is a certain point, beyond which, whatever be the means employed, 

 their temperature can never be elevated. Water, for instance, under 

 the common atmospherical pressure, may be heated up as far as 212° 

 of Fahrenheit ; but all the heat which we employ in the endeavour to 

 raise this temperature higher, is only dissipated in the ensuing evapora- 

 tion. In like manner, the substance constituting a comet must have a 

 certain point of its own, which, however near it may approach the sun, 

 its mean temperature can never exceed. The tail of the comet may be 

 expanded to a prodigious length, the nebulous envelope may become 

 enlarged to an equal extent ; even the materials on the surface of the 

 nucleus, by volatilization, may pass into a gaseous or aerial form ; but 

 the planetary or solid body itself will experience no accession of heat 

 beyond that point of maximum temperature, which its own nature and 

 constitution determine. 



" In the second place, we may observe, that when, by any means, 

 the density of bodies is made to change, by a process, whether of rare- 

 faction, on the one hand, or of condensation, on the other, they are al- 

 ways found to undergo a corresponding diminution or increase of tem- 

 perature. When, therefore, in the approach of a comet to the sun, all 

 the parts of its nebulous envelope and tail, which in the remoter regions 

 of its course had been gathered close about the head, become expanded 

 and attenuated, a very large proportion of the solar heat, which would 

 otherwise have passed into the nucleus, and contributed to raise its tem- 

 perature to a certain point, is carrie<l off by the envelope and tail, in 

 order to preserve an equilibrium among the several parts. Let us at- 

 tempt to form some estimate of the actual loss of temperature thus sus- 

 tained by the rarefaction. If we assume that the nebulous matter 

 is elevated about ,30 times its former height, the diminution of den- 

 sity, corresponding with the increase of volume, will amount to 

 (3€)^ or 27,000; and employing the tWmula given in the Supple- 

 ment to the EncyclopfiBdia Britannica, article ' Climate,' we l»ave 



