240 TOILERS IN THE SEA. 



"they play an important part in protecting the 

 upper surfaces of coral-reefs, behind, and within, the 

 breakers, where the true corals, during their outward 

 growth, become killed by exposure to the sun and 

 air." 



The corallum, or coral skeleton, which is secreted 

 by the coral animals, and enters into the composition 

 of coral-reefs and coral-rock, is almost identical with 

 ordinary limestone. It is somewhat harder than 

 limestone or marble, and rings when struck with a 

 hammer. " It is a common error," writes Dana, "to 

 suppose that coral, when first removed from the 

 water, is soft, and afterwards 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 pene- 

 trated 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." 

 Chemically, the coral-reefs are composed of carbonate 

 of lime, viz., they contain from ninety-five to ninety- 

 eight parts in one hundred of that constituent, 1 and 

 the coral animals are the medium, through which this 

 is abstracted from the sea water, and deposited in 



1 Madrepora cervicornis, contains in 100 parts 98*07 of car- 

 bonate of lime, o - 32 phosphate of lime, and 1*93 of water and 

 organic matters. American Journ. Sri.) iii. i. 168. 



