LIVING SUBSTANCE 101 



and certain metals, such as copper, which are occasionally found in 

 traces in living substance, perhaps possess no importance at all for 

 the vital processes of the organisms in which they have been 

 observed. 



But no one of all these organic elements is limited exclusively 

 to organic nature. 



Carbon occurs in the air, combined with oxygen, as carbonic 

 acid, and in large masses in the calcium carbonate of sedimentary 

 rocks. 



Hydrogen, likewise combined with oxygen, as water, covers the 

 greater part of the earth's surface. 



Oxygen occurs both free as a gas in the atmospheric air, of 

 which it constitutes about 21 per cent., and also combined with a 

 large number of other elements. 



Nitrogen occurs likewise both in the free state in the air, com- 

 prising about 79 per cent., and also combined with hydrogen and 

 oxygen in the compounds of ammonia, both ammonium nitrate 

 and nitric acid. 



Sulphur is wide-spread in combination with oxygen in sulphates. 



Phosphorus behaves similarly, and is to be found everywhere in 

 the phosphates of the alkalies and the alkaline earths. 



Chlorine is very widely distributed, combined with sodium, as 

 common salt. 



Potassium occurs in combination with chlorine as potassium 

 chloride, and with acids as nitrates, sulphates, and phosphates. 



Soditim, chiefly in the form of sodium chloride or common salt, 

 is found everywhere on the surface of the earth ; it is in solution 

 in the sea, in the earth, and forms large solid masses in salt strata. 



Magnesium is a constant accompaniment of' potassium and 

 sodium, and is similarly combined, occurring as magnesium chloride, 

 carbonate, sulphate, and phosphate. 



Calcium, in the form of calcium carbonate, silicate, sulphate, 

 and phosphate, occurs in the vast limestone strata of the sedi- 

 mentary rocks. 



Iron is very wide-spread over the earth's surface in the form 

 of sulphur compounds, oxides and their salts. 



Silicon appears almost exclusively combined with oxygen in the 

 form of silicic acid and its salts in igneous rocks. 



Fluorine occurs chiefly in combination with calcium as fluor 

 spar. 



Bromine and iodine are present in many salt strata, as well as 

 in sea-water, as sodium bromide (iodide), and potassium bromide 

 <(iodide). 



Aluminium is spread over the whole earth in combination with 

 oxygen as clay, and in the latter form, combined with silicic acid, 

 as feldspar. 



Manganese and all the other'metals that are observed occasionally 



