267 



ered the most impure water in use. This is more particularly the case 

 with the water from wells dug in a clay soil, and in large towns, where 

 surface filth accumulates in great abundance. 



WATER AS A SOLVENT. 



Jn what form of combination are these earthy salts held in solution ? 

 This is a question of some considerable practical importance, and one 

 with which all should be familiar. In some few cases, the water dis- 

 Bolves the substance directly, as silicia (sand) and sulphate of lime 

 (gypsum ;) while, in other cases, in order that the water should become 

 a solvent, it must contain an excess of carbonic acid gas, or the salt 

 must be in the form of a bi-carbonate. Thus pure water will dissolve 

 little or no carbonate of lime, (limestone,) but allow the water to take 

 carbonic acid, and convert the carbonate into a bi-carbonate, and it will 

 readily dissolve a hundredth part of the limestone. It becomes a "hard 

 water." By long exposure of this water to the air, or by boiling it, the 

 free carbonic acid escapes as a ga«, consequently, the water loses its 

 power of holding the lime in solution. It becomes turbid and milty, 

 and the lime is deposited. Hence, the calcarious deposits in the vicin- 

 ity of "hard water" springs, and the calcarious incrustations on the in- 

 side of boilers. This carbonic acid is derived, in part, from the atmos- 

 phere, and in part from the soil. In the decay of vegetable matter, 

 the carbon of the plant unites with the oxygen of the air, to form the 

 acid. Hence, we find it very abundant in open, porous soils, highly 

 charged with decaying organic matter. Water leached through such 

 soils would acquire carbonic acid, and, coming in contact with limestone 

 would dissolve it in great quantity. A "hard water" would be formed. 

 Boiling expels the carbonic acid, and the lime is no longer dissolved. 

 The water is made soft. If, however, the water ia made "hard" by the 

 presence of the sulphate of lime, the sulphate is not aftected by boil- 

 ing. In this case, the hme may be precipitated by the addition of 

 carbonate of soda or potassa. An insoluble carbonate of lime is formed 

 and precipitated, while the sulphuric acid unites with the soda. Hence, 

 the great practical importance of knowing the precise condition of the 

 lime held in solution — that is, whether it is a carbonate or sulphate. 



Waters holding in solution the sulphates are liable to a form of 

 spontaneous decomposition, highly deleterious to health, when they come 

 in contact with organic matter, either animal or vegetable. All animal 



