74 CORALS AND CORAL LSLANDS. 



fragment broken otf, dropping in a favourable place, would 

 become the germ of another coral plant, its base cementing 

 by means of new coral secretions to the rock on which it 

 might rest ; or,, if still in contact with any part of the parent 

 tree, it would be reunited and continue to grow as before. 

 The coral zoophyte may be levelled by transported masses 

 swept over it by the waves ; yet, like the trodden sod, it 

 sprouts again, and continues to grow and flourish as before. 

 The sod, however, has roots which are still unhurt ; while the 

 zoophyte, which may be dead at base, has a root — a source 

 'or centre of life — in every polyp that blossoms over its surface. 

 Each animal might live and grow if separated from the rest, 

 and would ultimately produce a mature zoophyte. 



V. COMPOSITION OF CORAL, 



Ordinary corals have a hardness a little above that of com- 

 mon limestone or marble. The ringing sound given, when 

 coral is struck with a hammer, indicates this superior hardness. 

 It is possible that it may be owing to the carbonate of lime 

 being in the state of aragonite, whose hardness exceeds a little 

 that of ordinary carbonate of lime or calcite. It is a common 

 error of old date to suppose that coral when first removed 

 from the water is soft, and afterward hardens on exposure. 

 For, in fact, there is scarcely an appreciable difference ; the 

 live coral may have a slimy feel in the fingers ; but if washed 

 clean of the animal matter, it is found to be quite firm. The 

 water with which it is penetrated may contain a trace of lime 

 in solution, which evaporates on drying, and adds slightly to 

 the strength of the coral ; but the change is hardly appreciable. 

 A branched Madrepore rings on being struck when first 

 collected ; and a blow in any part puts in hazard every branch 

 throughout it, on account of its elasticity and brittleness. 

 The specific gravity of coral varies from 2-5 to 2-8 : 2-523 was 

 the average from fifteen specimens examined by Prof. Silliman. 



Chemically, the common reef-corals, of which the branching 

 Madrepora and the massive Astr^eas are good examples. 



