ON AVATKR AND ITS COMPOUNDS 47 



Neverthrlos. in passing through the atmosphere, r;i in and snow succeed 

 in catcliinu' tin- .lust held in it, and dissolve air, which is found in every 

 water. The dissolved gases of the atmosphere are partly disengaged, 

 as bubbles from water on heating, and water after long boiling is quite 

 freed from them. 



In general terms water is called pure when it is clear and free from 

 insoluble particles held in suspension and visible to the naked eye, from 

 which it may be freed by nitration through charcoal, sand, or porous 

 (natural or artificial) stones, and when it possesses a clean fresh taste. 

 It depends on the absence of any tastable, decomposing organic matter, 

 on the quantity of air 5 and atmospheric gases in solution, and on the 

 presence of mineral substances to the amount of about 300 grams per 

 ton (or cubic metre, or, what is the same, 300 milligrams to a kilo- 

 gram or litre of water), and of not more than 100 grams of organic 

 matter. 6 Such water is suitable for drinking and every practical 



5 The taste of water is greatly dependent on the quantity of dissolved gases it con- 

 tains. On boiling, these gases are given off, and it is well known that, even when cooled, 

 boiled water has, until it has succeeded in absorbing gaseous substances from the atmo- 

 sphere, quite a different taste from fresh water containing a considerable amount of gas. 

 The dissolved gases, especially oxygen and carbonic anhydride, have an important 

 influence on the health. The following instance is very instructive in this respect. The 

 Grenelle artesian well at Paris, at the first period of its opening, supplied a water which 

 had an injurious effect on animals and people. It appeared that this water did not 

 contain oxygen, and in general was very poor in gases. As soon as it was made to fall in 

 a cascade, by which it absorbed air, it proved entirely fit for consumption. In long sea 

 voyages by steamer sometimes fresh water is not taken or only taken in a small quantity 

 because it spoils by keeping, and becomes putrid from the organic matter it contains under- 

 going decomposition. Fresh water may be obtained directly from sea-water by distilla- 

 tion. The distilled water 116 longer contains sea salts, and is therefore fit for consump- 

 tion, but it is very tasteless and has the properties of boiled water. In order to render it 

 palatable certain salts, which are usually held in fresh water, are added to it, and it is 

 made to flow in thin streams exposed to the air in order that it may become saturated 

 with the component parts of the atmosphere that is, absorb gases. 



6 Hard icat^r is such as contains much mineral matter, and especially a large pro- 

 portion of calcium salts. Such water, owing to the amount of lime it contains, does not 

 form a lather with soap, prevents vegetables boiled in it from softening properly, and 

 forms a great deal of incrustation on vessels in which it is boiled. Owing to its high 

 degree of hardness, it is injurious for drinking purposes, which is evident from the fact 

 that in many large cities the death-rate decreased after introducing a soft water in the 

 place of a hard water. Putrid water contains a considerable quantity of decomposing 

 organic matter, chiefly vegetable, but in populated districts, especially in towns, chiefly 

 animal remains. Such water acquires an unpleasant smell and taste, by which stagnant 

 bog water and the water of certain wells in inhabited districts are particularly charac- 

 terised. Such water is especially harmful at a period of epidemic. It may be partially 

 purified by passing through charcoal, which retains the putrid and certain organic sub- 

 stances, and also certain mineral substances. Turbid water may be purified to a certain 

 extent by the addition of alum, which aids, after standing some time, the formation of a 

 sediment. Condy's fluid (potassium permanganate) is another means for purifying 

 putrid water. A solution of this substance, even if very diluted, is of a red colour ; on 

 adding it to a putrid water, the permanganate oxidises and destroys the organic matter. 

 When added to water in such ;i quantity as to impart to it an almost imperceptible rose 



