62 Temperature of the Terrestrial Glohe, 



layers, while, by reason of the mobility of the liquid particles, 

 these quantities of heat would be carried to the surface, where 

 the whole would be dissipated, by radiation, through the sur- 

 rounding space. While thus passing to a solid state, the hquid 

 mass would lose all the heat developed by this change. But 

 this may be better understood in ascending to the probable cause 

 of the primitive fluidity of the planets. 



This we may illustrate by reasoning on the known hypothesis 

 of Laplace, upon the origin of these bodies, namely, that they are 

 portions of the sun's atmosphere, which it has successively aban- 

 doned, in concentrating itself around that body. The earth was, 

 then, primitively, an aeriform mass, of very great volume, rela- 

 tively to what it now is, and formed of the different materials, 

 solids and fluids, of which it is now composed, which were then 

 in a state of vapour, that is of an aeriform fluid, of which the 

 density could not surpass a given maximum, proportional to its 

 degree of heat, and which would become liquid or solid accord- 

 ing to the augmentation of the pressure it experienced, without 

 changing its temperature. That of the earth would depend, 

 then, upon the point it should occupy in space, and upon its dis- 

 tance from the sun ; and might be more or less elevated. But 

 independently of the attractions and repulsions which take place 

 only among particles near each other, and which produce the elas- 

 tick force of aeriform fluids, equal and contrary to the pressure 

 they sustain, the particles of the earth were also subject to their 

 mutual attraction, in the inverse ratio of the squares of their dis- 

 tances ; and from this force there resulted, upon all the layers of 

 the fluid mass, a pressure, nothing at its surface, increasing from 

 the surface to the centre, and which, at the centre itself, becomes 

 exceedingly great, surpassing 100,000 times, the weight of the 

 present atmosphere. It is this increasing pressure, and not an ex- 

 teriour temperature much lower than that of the fluid, which has 

 successively reduced all the layers to a solid state, commencing 

 with those at the centre, and continuing from one to another, 

 until nothing remained except the matter composing the oceans 

 and the atmosphere of the present day. But this reduction has 

 not been instantaneous ; for a certain period of time was neces- 

 sary for each layer to approach the centre, towards which it was 

 urged, by the pressure it experienced, and which was the exci- 

 ting cause of this movement. Now we may readily infer, when 



