WATER. 459 



different forms as drink: (1) Rain-water, which most closely resembles 

 distilled or chemically pure water, always contains minute quantities 

 of C0 9 NH 3 , nitrous and nitric acids. (2) Spring-water usually con- 

 tains much mineral substance. It is formed from the deposition of watery 

 vapour or rain from the air, which permeates the soil containing much 

 C0 2 ; the CO, is dissolved by the water, and aids in dissolving the 

 alkalies, alkaline earths, and metals, which appear in solution as 

 bicarbonates e.g., of lime or iron oxide. The water is removed from 

 the spring by proper mechanical appliances, or it bubbles up on the 

 surface in the form of a " spring." (3) The running-water of rivers 

 usually contains much less mineral matter than spring-water. Spring- 

 water floating on the surface rapidly gives off its C0 2 , whereby many 

 substances e.g., lime are thrown out of solution, and deposited as 

 insoluble precipitates. 



Gases. Spring-water contains little 0, but much CO.,, which latter 

 gives to it its fresh taste. Hence, vegetable organisms flourish in 

 spring-water, while animals requiring, as they do, much 0, are 

 but poorly represented in such water. Water flowing freely gives up 

 C0 2 , and absorbs from the air, and thus affords the necessary con- 

 ditions for the existence of fishes and other marine animals. Eiver- 

 water contains -^ - ^ of its volume of absorbed gases, which may be 

 expelled by boiling or freezing. 



Drinking-water is chiefly obtained from springs. River-water, if 

 used for this purpose, must be filtered to get rid of mechanically 

 suspended impurities. For household purposes a charcoal filter may 

 be used, as the charcoal acts as a disinfectant. Alum has a remarkable 

 action ; if '00 01 per cent, be added, it makes turbid water clear. 



Investigation of Drinking-water. Drinking-water, even in a thick 

 layer, ought to be completely colourless, not turbid, and without odour. 

 Any odour is best recognised by heating it to 50C., and adding a 

 little caustic soda. It ought not to be too hard i.e., it ought not to 

 contain too much lime (and magnesia) salts. 



By the term " degree of hardness" of a water is meant the unit amount of 

 lime (and magnesia) in 100,000 parts of water; a water of 20 of hardness con- 

 tains 20 parts of lime (calcium oxide) combined with C0 2 , sulphuric, or hydro- 

 chloric acids (the small amount of magnesia may be neglected). A good drinking 

 water ought not to exceed 20 of hardness. The hardness is determined by titrating 

 the water with a standard soap solution, the result being the formation of a 

 scum on the surface. 



The hardness of unboiled water is called its total hardness, while that of boiled 

 water is called permanent hardness. Boiling drives off the COg, and precipitates 

 the calcium carbonate, so that the water at the same time becomes softer. 



The presence of sulphuric acid, or sulphates, is determined by the water 

 becoming turbid on adding a solution of barium chloride and hydrochloric acid. 



Chlorine occurs in small amount in pure spring water, but when it occurs there 



