364 THE WORLD OF LIFE 



carbonate of lime (CaCO s ). It is quite possible, however, 

 that these are all results of animal secretions, as in coral- 

 reefs ; or of the debris of the hard parts of marine animals, as 

 in the Globigerina-ooze. Limestones exist among the oldest 

 rocks, but as we know that marine life was very much older, 

 this is no objection. All water holds in solution a large 

 quantity of carbonic acid gas, so that both air and water are 

 the source of the most essential elements for building up 

 the bodies of plants and animals. 



The ocean also holds a large amount of carbonate of 

 lime in solution, and this is kept permanently dissolved by 

 the large amount of carbonic acid gas always present, which 

 is sufficient to dissolve five times the amount of carbonate 

 of lime which actually exists. Deposits of inorganic lime- 

 stone are, therefore, now never formed except by long- 

 continued evaporation in isolated bodies of salt water. This 

 renders it more probable that all pure limestone rocks are 

 really very ancient coral-reefs consolidated and crystallised 

 by heat and pressure under masses of superincumbent 

 strata. 



The altogether remarkable and exceptional properties 

 of carbon are fully recognised by modern chemists, as well 

 shown by Professor H. E. Armstrong's statements in his 

 Presidential Address to the British Association in 1 909 : 



"The central luminary of our system, let me insist, is the 

 element carbon. The constancy of this element, the firmness of its 

 affections and affinities, distinguishes it from all others. It is only 

 when its attributes are understood that it is possible to frame any 

 proper picture of the possibilities which lie before us of the place of 

 our science in the cosmos." 



And a little farther on he says : 



" Our present conception is, that the carbon atom has tetrahedral 

 properties in the sense that it has four affinities which operate 

 practically in the direction of four radii proceeding from the centre 

 towards the four solid angles of a regular tetrahedron. . . . The 

 completeness with which the fundamental properties of the carbon 

 atom are symbolised by a regular tetrahedron being altogether 

 astounding." 



And again : 



