vi CARBONIC ACID AND THE CARBONATES 105 



"a small portion of lime-water be dropped into water 

 impregnated with fixed air, slight clouds are immediately 

 formed, occasioned by the saturation of the lime by the 

 fixed air; these clouds,, however, disappear upon gently 

 shaking the vessel, the lime being dissolved by the 

 superabundant fixed air " (Essays, p. 34). 



Bergman also recorded the fact that crystals of calcareous 

 spar (Fig. 23) were dissolved by water containing fixed air, and 

 that minute crystals of similar shape were sometimes deposited 



FIG. 23 CRYSTALS OF CALC-SPAR. British Museum (Natural History). 



when chalk was reprecipitated by the escape of the fixed air 

 which held it in solution. These observations were 

 important as affording a clue to the way in which crystalline 

 calc-spar may have been produced ; they also afforded a 

 satisfactory explanation of the formation of stalactites (Fig. 

 24) and stalagmites by the escape of fixed air from the 

 dripping water of caverns in limestone districts. 



The hardness of water due to salts of lime and 

 magnesia. The mineral matter held in solution by fixed 

 air makes a water " hard," i.e. the water gives a curd with 

 soap instead of a lather. As this hardness is removed 



