126 PHYSIOGRAPHY. [chap. 



Although the proportion of mineral matter held in solution 

 in Thames water, appears, from such an analysis as that just 

 cited, to be extremely small, it must yet be remembered 

 that, taking into consideration the great volume of the 

 Thames, the total quantity of matter removed in this way 

 from the land and carried seaward is something enormous. 

 Professor Prestwich, taking the daily discharge of the Thames 

 at Kingston at 1,250 million gallons, and the salts in solu- 

 tion at 19 grains per gallon, calculates that the quantity of 

 mineral matter carried down in solution, at that locality, 

 amounts to 3,364,286 pounds, or 1,502 tons, every twenty- 

 four hours ; or say, roughly, a ton a minute. Of this amount 

 about 1,000 tons will consist of carbonate of lime, and 238 

 tons, of sulphate of lime. The total quantity of saline 

 matter carried invisibly away by the Thames from its basin 

 above Kingston, will thus reach, in the course of a year, to 

 the enormous amount of 548,230 tons. 



Although it has been shown that river-water contains a 

 smaller proportion of saline matter than is present in spring- 

 water, it would yet be a great mistake to assume on this 

 ground that the river-water is more piire and wholesome. On 

 the contrary, the river-water, though poor in mineral matter, 

 is usually rich in organic impurities, and is therefore much 

 less fitted for drinking purposes. Most of the water from 

 deep wells and springs in the Thames basin contains the 

 merest trace of organic matter ; but the river itself derives 

 a large proportion of organic impurity from the decomposing 

 vegetable matter, spread over the large surface of country 

 which it drains. A more serious source of contamination, 

 however, is to be found in the sewage-matter, which is 

 allowed to be run into it from the centres of population 

 seated on or near its banks. It must, nevertheless, be borne 

 in mind that, by the constant exposure of fresh surfaces 



