io6 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



when the water is boiled it is called TEMPORARY HARDNESS. 

 Cavendish showed that the whole of the chalk in such a 

 water could be " precipitated ... by the addition of a 

 proper quantity of lime-water" (loc, cit, p. 107). This 

 method of removing the fixed air and so precipitating the 

 chalk from a hard water is used on a large scale in order to 

 " soften " the water-supply in chalky districts such as that of 

 the Caterham Valley. 



Magnesia, of which Cavendish detected a small quantity 



mixed with the 

 chalk from Rath- 

 bone Place water, 1 

 also owes its solu- 

 bility to fixed air. 

 It dissolves to a 

 larger extent than 

 chalk, and the 

 solution is used in 

 medicine as a 

 gentle alkali under 

 the name of " fluid 

 magnesia." Like 

 chalk, the mag- 

 nesia produces a 

 temporary hard- 

 ness, which can 

 be removed by 

 boiling, or by the 



addition of lime ; but the precipitation of the magnesia, and 

 the softening of the water, takes place much more slowly 

 than in the case of chalk. 



Gypsum or selenite (of which Cavendish collected 39 



1 By acting on the earthy precipitate with oil of vitriol the chalk 

 was converted into sparingly-soluble selenite or gypsum, and the 

 magnesia into Epsom salts, of which 18 grains were extracted. 



FIG. 24 STALACTITE, i.e., CHALK DEPOSITED BY THE 

 ESCAPE OF FIXED AIR FROM DRIPPING WATER. 

 British Museum (Natural History). 



