=Exp. with Lime-water.= Pour a little lime-water into a test-tube. With a small glass tube blow into it for a few minutes, when it will become milky; continue the blowing for a few minutes more, when it will lose its cloudy appearance and become clear again. The following explains this: in the first place there was forced into the lime-water, from the lungs, air containing an excess of carbonic acid; this united with the lime in solution in the water and formed carbonate of lime. Carbonate of lime is insoluble in water which contains no carbonic acid, or very little,[8] but will dissolve in water which is charged with it, and this is produced by the continued blowing. Now if this water be freed of its excess of carbonic acid by boiling, the carbonate of lime will be freed from its soluble state, and will fall as a precipitate and settle on the sides of the vessel. From this we learn that water may be freed from carbonate of lime in solution in it by boiling. =Organic Matter in Water.= There is another class of impurities in water of vastly more importance than either the atmospheric gases or lime. These are the organic substances which it always contains, especially that which has flowed over land covered with vegetation, or that which has received the drainage from sewers. The soluble matter found in such water is excellent food for many kinds of micro-organisms which often form, by their multiplication, poisons very destructive to animal life. Or the organisms themselves may be the direct producers of disease, as for instance the typhoid fever bacillus, the bacillus of cholera, and probably others which occur in drinking-water. These organisms are destroyed by heat, so that the most valuable effect produced in water by boiling it is their destruction. Such water is, therefore, a much safer drink to use than that which has not been boiled. Water should always be boiled if there is the slightest suspicion of dangerous impurities in the supply. =Use of Tea and Coffee.= This leads us to the thought that the extensive use of tea and coffee in the world may be an instinctive safeguard against these until recently unknown forms of life. The universal use of cooked water in some form in China is a matter of history. The country is densely populated, the sewage is carried off principally by the rivers, so that the danger of contracting disease through water must be very great, and it is probable that instinct or knowledge has prompted the Chinaman to use but very little water for food except that which has been cooked. Whatever the reason, the custom is a national one. The every-day drink is weak tea made in a large teapot and kept in a wadded basket to retain the heat; the whole family use it. The very poor drink plain hot water or water just tinged with tea.