176 LIGHT SCIENCE FOR LEISURE HOURS. 



(one could suppose) to an extraordinarily high tempera- 

 ture. There have been cases where comets have been 

 so near to the sun as to account for almost any con- 

 ceivable change in the constitution of their elements. 

 An intensity of heat of which we can form no concep- 

 tion must have been experienced, for example, by 

 Newton's comet ; and a still fiercer heat dissipated the 

 substance of the comet of 1843. But Winnecke's 

 comet at the time of observation was at far too great a 

 distance from the sun for us to assign to its mass a 

 temperature which under ordinary circumstances would 

 account for the volatilisation of carbon. 



Nor does the rarity of the atmosphere in which the 

 comet was moving serve to help us in our difficulty. 

 Doubtless we are little familiar with the effects which 

 terrestrial elements would experience if they were dis- 

 tributed freely in the ether occupying the interplanetary 

 spaces. But so far as our experience enables us to 

 judge, we should rather look for intensity of cold than 

 of heat under such circumstances. We see the heights 

 of the Andes and of the Himalayas clothed in perpetual 

 snow, though day after day the fierce heat of the tropi- 

 cal sun pours down upon them, and though there is no 

 winter there (in our sense of the word) during which 

 the snows are accumulated. We know that the explana- 

 tion of this peculiarity lies in the extreme rarity of the 

 air at a great height. It seems, therefore, reasonable 

 to conclude that the cold of the interplanetary spaces 

 must be far greater. Yet here we have an object whose 

 light comes from the incandescent vapour of so fixed 



