COMMON THINGS. WATER. 



water for marine stores on this account, the more pure river water 

 fermenting less rapidly, and remaining more or less foul and 

 putrid for a much longer time. 



For the supply of London, however, where this spontaneous 

 purification is not to be waited for, it is obvious that the water 

 should be taken from that part of the river above Richmond which 

 is beyond the influence of the tides, and where it is not liable to 

 be polluted by the contents of the sewers, the offal of manufac- 

 tories, and the mud stirred up by steamers. 



28. Water was supposed by the ancients to be one of the 

 elements or simple substances of which all others are composed. 

 It was ascertained, however, towards the close of the last century, 

 that it is a compound of two substances as different in their form 

 and properties from water itself as can well be imagined. Water 

 is a heavy liquid. Its constituents are light gases, one of them 

 being the lightest material substance ever yet discovered. Water 

 is an antagonist of fire. One of its constituents is the most highly 

 combustible substance in nature, and the other is a gas whose 

 presence is necessary to fire, and hence called a supporter of com- 

 bustion. In order to demonstrate the composition of water, it is 

 necessary, in the first instance, to obtain that liquid absolutely pure, 

 and it has been already stated that it is never so found naturally. 



29. All fixed air with which water is charged may be dismissed 

 from it by boiling ; but to separate it from such matters as it 

 may hold in solution, it must be submitted to the process of 



DISTILLATION. 



30. The principle of distillation is easily explained. 



If water which holds in solution any earthy or saline substance 

 be raised to its boiling point, it will be converted into vapour, but 

 the substance it holds in solution will not be so converted. As 

 the water is gradually evaporated, the substance held in solution 

 remaining undiminished, the solution first is rendered stronger 

 and more concen'Tated, inasmuch as the same quantity of saline 

 matter is dissolved in a less quantity of water. As the process 

 goes on, the entire quantity of water will at length be evaporated, 

 and the earthy or saline matters which it held in solution will 

 remain in the vessel in which the evaporation takes place. This 

 is an experiment which may be tried by any person. Let a table- 

 spoonful of water, in which salt has been dissolved, be held for 

 some minutes over the flame of a spirit lamp. The liquid will 

 boil, and will soon be entirely converted into vapour, the salt alone 

 remaining in the spoon. 



31. But when it is the object, as in distillation, to obtain, not the 

 matters held in solution by the water, but the pure water itself 

 separated from these matters, it is necessary to prevent the vapour 



104 



