292 CLASSICS OF MODERN SCIENCE 

 speedy union of the masses, that the larger part of the heat must 

 have been diffused by radiation into space, before the masses could 

 form bodies possessing the present density of the sun and planets, 

 and that these bodies must once have been in a state of fiery fluidity. 

 This notion is corroborated by the geological phenomena of our planet ; 

 and with regard to the other planetary bodies, the flattened form of 

 the sphere, which is the form of equilibrium of a fluid mass, is in- 

 dicative 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 invaded. 

 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 light 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 on her orbit — which is not to be feared 

 in the existing arrangements 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 

 unfavourable 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 therefore be quite fused and for 

 the most part reduced to vapour. 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 developed by the shock would 

 be four hundred 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, fire-balls, 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 fall 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 hot, the friction was long ago thought of which 

 they experience in passing through the air. \\^e can now calculate 

 that a velocity of 3,000 feet a second, supposing the whole of the f ric- 



