vi CARBONIC ACID AND THE CARBONATES 107 



grains by evaporating 494 ounces of the water to a bulk of 

 three ounces) shares with all the soluble salts of lime the 

 property of rendering water hard. As the gypsum is not 

 precipitated when the water is boiled, the hardness which it 

 produces is called PERMANENT HARDNESS. This permanent 

 hardness can be removed by adding to the water a mild 

 alkali such as soda or potash, which precipitates the lime in 

 the form of chalk and so renders the water soft. This is 

 the method generally used for softening permanently hard 

 waters. 



Soluble salts of magnesia (of which Cavendish precipitated 

 36 grains by the addition of fixed alkali) also contribute to 

 the permanent hardness of water. Sea water, which contains 

 a little gypsum, and a relatively large proportion of Epsom 

 Salts, is so hard that it is practically impossible to make it 

 lather, even by using a very large amount of soap. The 

 precipitation of soap by the mineral matter present in hard 

 waters was discussed by Bergman (".Of the Analysis of 

 Waters," Essays, pp. 90-192), who made use of a solution 

 of soap in alcohol as a means of detecting such substances 

 in water, thus, 



"Soap is not soluble in every kind of water; this is 

 occasioned either by a disengaged acid, or by a large pro- 

 portion of ... salt with an earthy or metallic base . . . : 

 such waters are generally called hard waters, and are unfit 

 for washing cloths, as also for boiling pulse, and the harder 

 kinds of flesh." 



" If there be present in a can of water but 8 grains of 

 alum, [muriate of] magnesia or [muriate of] lime, a single 

 drop of this water occasions a turbidness in a solution of 

 soap in alcohol, diluted with an equal bulk of distilled 

 water" (Essays, p. 139). 



Lane (1769) discovers that fixed air will dissolve 

 iron. Fixed air also possesses the power of dissolving iron, 

 giving rise to chalybeate waters. In these waters the iron 



