THE 



UNIVERSITY 



- 

 CALCIUM MAGNESIUM. 15 



Calcium is an extremely abundant element, always occurring 

 in combination. The carbonate, constituting the main ingre- 

 dient in limestone, chalk, and marble, and the sulphate, 

 which is found as gypsum or sclenite, CaSO 4 .2H. 2 O, and 

 also as anhydrite, CaS0 4 , are very abundant. Calcium is 

 also found in union with phosphoric acid in the various 

 deposits of apatite, 3 [Ca 8 P.,0 H ] CaCl. 2 (or CaF 2 ). Calcium 

 carbonate, which is extremely abundant, dissolves in water 

 containing carbonic acid and is therefore found in all natural 

 waters, from which it is extracted by shell-fish, &c., and forms 

 the chief constituent of their hard parts. It is essential as a 

 plant food, but its agricultural importance arises rather from 

 its effect in altering the texture of soils and in modifying the 

 chemical changes attending the fermentation and decay of their 

 organic matter. For example, calcium carbonate, lime, and 

 other calcium compounds have a remarkable action upon clay, 

 rendering it much less tenacious and plastic. The presence of 

 calcium carbonate or some other substance capable of acting 

 as a weak base is essential to the important process of nitrifi- 

 cation, Into plants calcium is probably absorbed in the form 

 of nitrate, phosphate, sulphate, or carbonate, and is found in 

 all parts of the organism. In animals the calcium compounds 

 are usually concentrated largely in the hard parts, the bones 

 or shells. 



Owing to its abundance, calcium is rarely used as a manure 

 in the strict sense of the word, i.e., as a plant food, but it is 

 largely used in agriculture either for the sake of the improve- 

 ments it produces in the texture of the soil as free lime or in 

 combination with other ingredients of manurial value, e.g., 

 phosphoric acid, as basic slag, superphosphates, &c. 



Magnesium also occurs only in a state of combination, often 

 associated with calcium. Limestone and other forms of car- 

 bonate of lime invariably contain some carbonate of magnesium, 

 which in some, e.g., magnesian limestone, is present in con- 

 siderable proportion. 



Magnesium is also found in many silicates, e.g., meerschaum, 

 steatite, talc, and serpentine. It is also present in sea-water 

 and in many mineral springs, to which it imparts a bitter taste. 



