486 THE SEDIMENT OF WATER. 



lastly, it is the property of standing waters to hold 

 in solution, without deposit, matters held in chemical 

 solution, as for instance, say a certain quantity of 

 common salt. Chemistry, however, teaches us, that 

 water will only take up a certain fixed quantity of 

 such matters. When the maximum percentage is thus 

 reached, it is technically said to contain a " saturated 

 solution," and then, the moment evaporation sets in, 

 so that a portion of the water is abstracted as vapour, 

 " precipitation " takes place : that is to say the surplus 

 matter falls to the bottom in the form of a sediment. 

 A common illustration of this is seen in the sediment 

 deposited on a kettle in which water has been boiled, 

 which very generally consists principally of carbonate 

 of lime; but the same result, we need hardly say, will 

 follow when water evaporates, after standing for a 

 certain length of time in a pond or lake, or in a water 

 jug. From the clearest drinking water lime will often be 

 deposited in this way upon the glass, to which it adheres 

 so firmly that it is sometimes very hard to remove; 

 and when the water contains vegetable or other organic 

 matter, a dark coloured sediment will be observed at 

 the bottom of the vessel. The reader will perhaps 

 pardon our going into details of this technical nature, 

 for as we shall proceed to show, a great deal depends 

 upon them, these matters being of vast importance in 

 the economy of Nature. 



The amount of solid material carried out to sea by 

 a great river in the form of silt, during the course of 

 a single year, would hardly be credited by those who 

 have not made such subjects a special study. Let us 

 take the case of the Thames, a very minor river, with 

 a drainage area of only 6,000 square miles, as an 



