76 CORALS AND CORAL LSLANDS. 



gypsum, and 5*597 sulphate of magnesia, = 100. This cor- 

 responds to about 16^ parts of sulphate of lime to 10,000 of 

 water. 



Fluorine has also been detected in sea-water ; so that all 

 the ingredients of coral are actually contained in the waters of 

 the ocean. 



It has been common to attribute the origin of the lime of 

 corals to the existence of carbonic-acid springs in the vicinity 

 of coral islands. But it is an objection to such a hypothesis, 

 that, in the first place, the facts do not require it ; and, in the 

 second, there is no foundation for it. The islands have been 

 supposed to rest on volcanic summits, thus making one 

 hypothesis the basis of another. Carbonic-acid springs are by 

 no means a universal attendant on volcanic action. The 

 Pacific affords no one fact in support of such an opinion. 

 There are none on Hawaii, where are the most active fires in 

 Polynesia ; and the many explorations of the Society and 

 Navigator Islands have brought none to light. Some of the 

 largest reefs of the Pacific, those of Australia and New 

 Caledonia, occur where there is no evidence of former vol- 

 canic action. 



The currents of the Pacific are constantly bringing new 

 supplies of water over the growing coral beds, and the whole 

 ocean is thus engaged in contributing to their nutriment. 

 Fish, mollusks, and zoophytes are thus provided with earthy 

 ingredients for their calcareous secretions, if their food fails of 

 giving the necessary amount ; and, by means of the powers of 

 animal life, bones, shells, and corals alike are formed. 



The origin of the lime in solution throughout the ocean is 

 an inquiry foreign to our present subject. It is sufficient here 

 to show that this lime, whatever its source, is adequate to 

 explain all the results under consideration. 



II. HYDROIDS. 



The annexed sketch represents a Hydra as it often occurs 

 attached to the under surface of a floating leaf — that of a 

 species of Lemna. The animal is seldom over half an inch 



