WATER AND ITS PURIFICATION 255 



others are dissolved only by water holding carbonic acid derived 

 from the air. 



Thus while silica, alumina, and minerals in which these sub- 

 stances predominate as constituents are practically insoluble in 

 water, common salt, Epsom salt (magnesium sulphate), and 

 gypsum (calcium sulphate) are more or less soluble in water, the 

 two last giving rise to the quality commonly called permanent 

 hardness in many natural waters. On the other hand, the 

 carbonates of lime and magnesia are practically insoluble in 

 pure water, but they are found as constituents of many natural 

 waters derived from springs, lakes, and rivers, owing to the 

 presence of carbonic acid, by which they are taken up, forming 

 unstable bicarbonates which are decomposed by boiling the 

 water. The presence of these compounds gives rise to temporary 

 hardness. Both these forms of hardness are the cause of some 

 discomfort in washing and destroy soap, but, at least in moderate 

 amount, they are certainly not injurious to health. They are, of 

 course, mischievous when used in steam boilers, as they give rise 

 to calcareous deposits and incrustations. 



With regard to the organic substances found in water, much of 

 this material is derived from the decay of vegetable matter, and 

 it is commonly the cause of the various shades of green, yellow, 

 or brown which are observable in the waters of streams and lakes. 

 It is especially noticeable in water which has flowed or soaked 

 through beds of peat, and may occur in water which, having 

 passed only over hard silicious rocks, is comparatively free from 

 saline or earthy impurity, and is therefore soft. Organic matter 

 of this kind in moderate amount, as it occurs in the majority of 

 waters supplied to communities, is not known to be definitely 

 harmful as a constituent of drinking water. Down to com- 

 paratively recent times much ingenuity was expended in devising 

 processes for estimating the amount of such substances in 

 drinking water, but it is now recognised that the dangerous 

 constituents in water are living organisms, and that the presence 

 of much decomposing nitrogenous organic matter is significant 

 chiefly as pointing to the probable contamination of the water 

 with animal excreta. The last may be derived from influx of 

 sewage, surface drainage from land supplied with manure or 

 other similar sources. The examination of water with the object 

 of determining whether or not it is fit for drinking by human 

 beings is therefore now dependent less on chemical analysis 



