178 ox THE IXTERACTIOX OF NATURAL FORCES. 



tened form of the sphere, which is the form of equili- 

 brium of a fluid mass, is indicative of a former state of 

 fluidity. If I thus permit an immense quantity of heat 

 to disappear without compensation from our system, the 

 principle of the conservation of force is not thereby in- 

 vaded. Certainly for our planet it is lost, but not for the 

 universe. It has proceeded -outwards, and daily proceeds 

 outwards into infinite space ; and we know not whether 

 the medium which transmits the undulations of liglit 

 and heat possesses an end where the rays must return, or 

 whether they eternally pursue their way through infinitude. 



The store of force at present possessed by our system is 

 also equivalent to immense quantities of heat. If our 

 earth were by a sudden shock brought to rest in her orbit 

 — ^which is not to be feared in the existing arrangement 

 of our system — by such a shock a quantity of heat would 

 be generated equal to that produced by the combustion of 

 fourteen such earths of solid coal. Making the most un- 

 favourable assumption as to its capacity for heat — that 

 is, placing it equal to that of water — the mass of the earth 

 would thereby be heated 11,200 degrees; it would, there- 

 fore, be quite fused, and for the most part converted into 

 vapom'. If, then, the earth, after having been thus 

 brought to rest, should fall into the sun — which, of 

 course, would be the case — the quantity of heat deve- 

 loped by the shock would be 400 times greater. 



Even now from time to time such a process is repeated 

 on a small scale. There can hardly be a doubt that 

 meteors, fireballs, and meteoric stones are masses which 

 belong to the universe, and before coming into the 

 domain of our earth, moved like the planets round the 

 sun. Only when they enter our atmosphere do they 

 become visible and fa-U sometimes to the earth. In order 

 to explain the emission of light by these bodies, and the 

 fact that for some time after their descent they are very 



