COMMON THINGS. WATER. 



little earthy matter in solution, is called, on the contrary, soft water. 

 Bain water and river water is in general soft, although the latter 

 is never free from some portion of earthy combination. 



20. Mineral springs are examples of water holding peculiar 

 mineral salts in solution, in quantities so considerable and of 

 qualities so peculiar as to render it altogether unfit for common use. 

 It acquires, however, from these, peculiar medicinal virtues. 



21. "Water generally holds suspended in it various impurities 

 which are not dissolved in it. Muddy water is an extreme 

 example of this. But without being actually muddy, water often 

 has many impurities, suspended without being dissolved in it. AIL 

 such impurities are removed by FILTEATIOX. 



22. In chemical researches, where the quantities of liquid operated 

 on are usually small, a species of paper, called filtering paper, is 

 used. This is white unsized paper, which is formed into a conical 

 bag, and placed in a glass funnel of corresponding shape. The 

 liquid to be filtered is made to pass slowly thsough the pores of the 

 paper, by which it is strained of the foreign matter suspended in it. 



23. The filters used in the arts and in domestic economy for the 

 purification of water have been very various. An open grained 

 stone from Teneriffe was formerly much used for this purpose, as 

 also porous unglazed earthenware. These have been more recently, 

 however, completely superseded by a variety of artificial filtering 

 apparatus, which for the most part consist of strata of gravel, sand, 

 and charcoal powder, through which the foul water is pressed by 

 its own weight, and by which it is very effectually strained of its 

 solid impurities. 



24. It has been stated that water is transparent and colourless ; 

 and, so far as respects any moderate quantity of the liquid which 

 is submitted to observation, this is true. But, strictly speaking, 

 water is neither absolutely transparent nor absolutely destitute of 

 colour. If we look into the sea, where the water has any consider- 

 able depth, we find that its colour is a peculiar tint of blue ; but 

 if, however, we take up a glass of the water, which thus appears 

 blue, we shall find it limpid and colourless. The reason of this 

 is, that the quantity of water contained in the glass reflects to the 

 eye too small a quantity of the colour to be perceivable ; while the 

 great mass of water viewed when we look into the deep sea, 

 throws up the colour in such abundance as to produce a strong 

 and decided perception of it. 



The same is true of all transparent coloured liquids. Sherry in 

 a decanter has a deep golden colour. Seen through the thin 

 stem of a tapering champagne glass it appears paler and paler, 

 until towards the point of the cone it loses all colour. 



It is probable that the colour of water arises partly from the 

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