22 ESSENTIALS OF BIOLOGY 



Oxygen 65 per cent., carbon 18-25, hydrogen 10, nitrogen 2-5, 

 calcium 1-5, phosphorus o-8, potassium 0-27, sodium 0-26, 

 chlorine 0-25, sulphur 0-24, magnesium 0-04, iron 0-02, all 

 other elements 0-87 per cent. This may be taken to represent 

 approximately the ultimate chemical composition of the body 

 of a vertebrate animal. Such animals differ chemically in respect 

 of (i) the percentage of water, (2) that of the mineral part 

 of the skeleton and (3) the relative mass of horny (proteid) 

 matter. 



Other animals differ from vertebrates mainly with respect to 

 the nature of the chemical substances that compose the skeleton : 

 these may be (i) lime and magnesia carbonates and phosphates ; 

 silica ; strontium carbonate or phosphate. The percentages of 

 water in the body also differ, being about 90-95 per cent, (in the 

 cases of some jelly-fishes) to about 50-60 per cent, (in vertebrates, 

 etc.). 



Thus our knowledge of the chemistry of inorganic and organic 

 things, earthly and celestial (in the cases of inorganic things, of 

 course), shows that : 



No chemical element occurs exclusively in the bodies of organisms. 



"jC. Water as a Natural Thing. Water exists mainly in 

 the ocean and atmosphere. It is a constituent of all inorganic 

 (mineral) materials, even of molten magma or of volcanic lava. 

 It is present to the extent of about 50 per cent, or more, in general, 

 in the bodies of animals and plants. It is an almost universal 

 solvent, so that almost all elements occur, in some form, in sea 

 water. Most chemical substances combine with it when they 

 crystallize. Most substances that dissolve in it dissociate into 

 sub-compounds that are chemically more reactive. A very great 

 number of chemical reactions only proceed in the presence of 

 water. 



All the characteristic substances found in the bodies of plants 

 and animals are only organically significant because they contain 

 half or more of their weight of water. 



Water is, therefore, a general medium in which both organic 

 and inorganic processes occur. Its presence is characteristic 

 alike of the organic and inorganic physico-chemical systems that 

 we know. We may therefore assume its general occurrence and 

 consider the characteristic chemical elements apart from it that 

 are present in organic and inorganic things. 



