254 CHEMICAL DISCOVERY AND INVENTION 



readily regenerated by passing over it a stream of producer gas or 

 any gas containing hydrogen or hydrocarbon vapours. A simple 

 arrangement would therefore be to provide two cylindrical 

 vessels filled with scrap copper maintained at a red heat, the one 

 being supplied with air, while the other is fed with producer gas 

 alternately. 



The remarkable chemically active form of nitrogen gas dis- 

 covered by Professor Strutt has been described (p. 121). 

 It seems not improbable that its properties may be hereafter 

 turned to account in connection with the fixation of nitrogen 

 from air. 



CHAPTER XVI 



WATER AND ITS PURIFICATION 



THE provision of a sufficient safe and suitable supply of water 

 has always been a subject of great public importance. But it is 

 only within the memory of the present generation that the 

 character of the impurities occurring in water used for drinking 

 has been completely understood, and that due precautions have 

 been taken in the selection and treatment of water to be supplied 

 to towns for all purposes. Water as it falls from the skies in the 

 form of rain, snow, and hail may be said to be, from the dietetic 

 point of view, pure, that is, it contains in solution only a small 

 quantity of the gases of the atmosphere. This is true of rain 

 water falling in the country, but as is well known the rain in 

 towns is always contaminated with soot and with acids, which 

 are the result of burning coal containing sulphurous and arsenical 

 minerals, to say nothing of acid impurities emitted from works 

 where chemical operations, such as alkali, glass, or cement 

 making are carried on. The water supplies are, however, always 

 drawn from districts as remote as possible from influences of this 

 kind, and are subject only to sources of contamination provided 

 by nature, and dependent chiefly on the geological character of 

 the strata through which the water rises or over which it flows. 

 The impurities thus naturally introduced are of two kinds, 

 namely, the inorganic and the organic. With regard to the 

 former we have to remember that some saline or earthy matters 

 are soluble in water without any addition or assistance, while 



