VIII.] COMPOSITION OF NATURAL WATERS. 129 



water weighs i,coo lbs. the same measure of water from 

 Margate will weigh 1,027 lbs. As a consequence of this 

 high density, it is easier to swim on salt than on fresh 

 water. Hence, too, the fresh water carried down by a river 

 tends to float for a time upon the surface of the dense sea- 

 waler; and, off the mouths of some great rivers, the water 

 is found to be nearly fresh for some distance out to sea. 



From the vast surface exposed by the sea, water is con- 

 tinually being evaporated by the aid of solar heat. But it is 

 [practically pure water which is thus drawn up into the 

 atmosphere, the saline constituents of the sea water being 

 left behind. Pure water condenses from this vapour, and 

 fi\lling upon the land as rain, charged to a certain extent with 

 the constituents of the atmosphere, it washes out more or less 

 of the soluble constituents of the rocks, which are ultimately 

 carried down to the sea, where they accumulate. There is 

 consequently, a never-ceasing transference of solid matter 

 from the land to the ocean — a transference, however, which 

 entirely escapes cognizance by the sight, since the matter 

 is carried down in a state of invisible solution. But, as 

 was remarked at the commencement of this chapter, in 

 addition to the dissolved mineral matter which thus eludes 

 observation, the Thames, like other rivers, bears a vast 

 quantity of other solid matter in a slate of mechanical 

 suspension, and therefore readily recognized by the eye. 

 This mechanical transport of solid matter from earth to sea 

 will form the subject of the next Chapter. 



K 



