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220 THE POPULAR SCIENCE MONTHLY. 



bacco-pipe. Presently it will become turbid. Continue tbe blowing, 

 and tbe turbidity will increase up to a certain degree of milkiness. 

 Go on blowing witb " commendable perseverance," and an inversion 

 of effect will follow : the turbidity diminishes, and at last the water 

 becomes clear again. 



The chemistry of this is simple enough. From the lungs a mix- 

 ture of nitrogen, oxygen, and carbonic acid is exhaled. The carbonic 

 acid combines with the soluble lime and forms a carbonate of lime 

 which is insoluble in mere water. But this carbonate of lime is to a 

 certain extent soluble in water saturated with carbonic acid, and such 

 saturation is effected by the continuation of blowing. 



Now take some of the lime-water that has been thus treated, place 

 it in a clean glass flask, and boil it. After a short time the flask will 

 be found incrusted with a thin film of something. This is the carbon- 

 ate of lime, which has been thrown down again by the action of boil- 

 ing in drawing off its solvent, the carbonic acid. This crust will 

 effervesce if a little acid is added to it. 



In this manner our tea-kettles, engine-boilers, etc., become incrusted 

 when fed with calcareous waters, and most waters are calcareous ; 

 those supplied to London, which is surrounded by chalk, are largely 

 so. Thus the boiling or cooking of such water effects a removal of its 

 mineral impurities more or less completely. Other waters contain 

 such mineral matter as salts of sodium and potassium. These are not 

 removable by mere boiling. 



Usually we have no very strong motive for removing either these 

 or the dissolved carbonate of lime, or the atmospheric gases from 

 water, but there is another class of impurities of serious importance. 

 These are the organic matters dissolved in all water that has run over 

 land covered with vegetable growth, or, more especially, which has 

 received contributiono from sewers or any other form of house-drain- 

 age. Such water supplies nutriment to those microscopic abomina- 

 tions, the mirococci, bacilli, bacteria, etc., which are now shown to be 

 connected with blood-poisoning possibly do the whole of the poison- 

 ing business. These little pests are harmless, and probably nutritious, 

 when cooked, but in their raw and wriggling state are horribly prolific in 

 the blood of people who are in certain states of what is called " recep- 

 tivity." They (the bacteria, etc.) appear to be poisoned or somehow 

 killed off by the digestive secretions of the blood of some people, and 

 nourished luxuriantly in the blood of others. As nobody can be quite 

 sure to which class he belongs, or may presently belong, or whether the 

 water supplied to his household is free from blood-poisoning organ- 

 isms, cooked water is a safer beverage than raw water. 



The requirement for this simple operation of cooking increases 

 with the density of our population, which on reaching a certain degree 

 renders the pollution of all water obtained from the ordinary sources 

 almost inevitable. 



