728 The Outline of Science 



are rare. Those that are always present are hydrogen, carbon, 

 oxygen, nitrogen, phosphorus, sulphur, potassium, magnesium, 

 calcium, and iron, and to these there should probably be added 

 sodium, chlorine, and silicon. Not uncommon are iodine (e.g. in 

 brown s'eaweeds and the thyroid gland), manganese (a trace in 

 most animals and in some plants), bromine (in brown seaweeds, 

 a trace in some animals) , and flourine (in bones and a few plants) . 



The first big fact is that the elements invariably present in 

 organisms are common in the inorganic world. The second big 

 fact is that the essential and most important elements in living 

 creatures are hydrogen, carbon, oxygen, and nitrogen. In most 

 animal and plant proteins sulphur is also present in addition to 

 the Big Four. In nucleo-proteins, like the chromatin of the nuclei 

 of all cells, there is likewise phosphorus. 



As far as the constituent elements go, there is nothing 

 peculiar in living matter. On or near the surface of the cooling 

 earth there would be a natural abundance, the chemists tell us, 

 of water and CO2, and in this triumvirate there is a unique en- 

 semble of fitnesses for forming the stones and mortar of the 

 house of life unique, for instance, in the synthetic possibilities 

 of carbon, in the solvent power of water, in the stimulating power 

 of oxygen (set free by the dislocation of COa) , and in the "attrac- 

 tive" capacity of all three drawing in other elements, such as 

 magnesium and iron, to serve fresh purposes. 



It may be of interest to notice individually some of the com- 

 mon elements that occur in living creatures. Hydrogen "ions" 

 are very important in respiration and gastric digestion. Oxygen 

 is a releaser of energy in vital combustion and an attractive agent 

 bringing in other useful elements. Water (HzO) is the chief con- 

 stituent (over 70 per cent.) of living matter. Nitrogen combines 

 with carbon, hydrogen, oxygen, and a little sulphur to form pro- 

 teins, and all living implies the building up and breaking down 

 of proteins. It gives living matter its restless explosiveness, 

 "because it is loath to combine with and easy to dissociate from 



