THE EARTH. 20S 



supposes, " that the sea's saltness is supplied not 

 only from rocks or masses of salt at the bottom 

 of the sea, but also from the salt which the rains 

 and rivers, and other waters, dissolve in their pas- 

 sage through many parts of the earth, and at 

 length carry with them to the sea."* But as 

 there is a difference in the taste of rock-salt 

 found at land, and that dissolved in the waters 

 of the ocean, this may be produced by the plenty 

 of nitrous and bituminous bodies that, with the 

 salts, are likewise washed into that great recep- 

 tacle. These substances being thus once carried 

 to the sea, must for ever remain there ; for they 

 do not rise by evaporation, so as to be returned 

 back from whence they came. Nothing but the 

 fresh waters of the sea rise in vapours ; and all the 

 saltness remains behind. From hence it follows, 

 that every year the sea must become more and 

 more salt ; and this speculation Dr Halley carries 

 so far as to lay down a method of finding out the 

 age of the world by the saltness of its waters. 

 " For if it be observed,"! says he, " what quantity 

 of salt is at present contained in a certain weight 

 of water, taken up from the Caspian Sea, for exr 

 ample, and, after some centuries, what greater 

 quantity of salt is contained in the same weight 

 of water taken from the same place ; we may 

 conclude, that in proportion as the saltness has 

 increased in a certain time, so much must it have 

 increased before that time ; and we may thus, by 

 the ride of proportion, make an estimate of the 



* Boyle, vol. iii. p. 221. f Phil. Trans, vol. v. p. 218. 



