TEXTURE AND COMPOSITION OF CORALS. 57 



hundred parts of the same, subjected to exact analysis, gave the 

 following result : — 



Silica, 22-00 



Lime, ...... 1303 



Magnesia, . . . . . 7*66 



Fluorid of calcium, . . 7 83 



Fluorid of magnesium, . . 12*48 



Phosphate of magnesia, . . . 2*70 



Alumina (and iron), . . . 16*00 



Oxyd of iron, 1830 



In other analyses similar results were obtained, with sometimes 

 a larger proportion of fluorids. 



The horny corals, (axes of Gorgonise and Antipathi,) were found 

 by Hatchett to have nearly the constitution of ordinary horn.* 



The sea-water and the ordinary food of the polyps are evi- 

 dently the source from which the ingredients of coral are ob- 

 tained. As coral is an animal secretion, there is no good reason 

 for the surprise with which this subject is sometimes approached. 

 The same powers of elaboration which exist in other animals 

 belong to polyps; for this function, as we have remarked, is the 

 lowest attribute of vitality. 



Neither is it at all necessary to inquire whether the lime in sea- 

 water exists as carbonate or sulphate, or whether chlorid of cal- 

 cium takes the place of these. The powers of life may make 

 from the elements present whatever results the functions of the 

 animal require. f 



Various waters were collected in the vicinity of the coral isl- 

 ands, and at different distances from them, for the purpose of 

 analysis in order to compare the constitution of the sea in differ- 

 ent parts ; but they were lost with the Peacock on the bar of the 

 Columbia river. The proportion of lime salts which occurs in the 

 water of the ocean is about ^\ to ^ T of all the ingredients in so- 

 lution. Prof. Forchhammer has ascertained that around the 

 West Indian seas, where corals abound, lime is not as abundant 

 as elsewhere in the ocean, the proportion, according to five analy- 



* Report on Zoophytes, p. 56. 



f If a drop of sea-water be slowly evaporated under a microscope of high power, 

 crystals of selenite (sulphate of lime) are produced, hav- 

 ing the annexed forms, the most common presented by 

 native crystals of this mineral, as stated in works on min- 

 eralogy. On adding more water, they are again dis- 

 solved ; and this may be repeated indefinitely. These re- 

 sults would seem + o indicate that the lime was mostly in 

 the state of a sulphate. 



Mr. Darwin states the remarkable fact, described by 

 Mr. Webster, (Voyage of the Chanticleer, ii, 319.) that a 

 deposit of salt and gypsum two feet thick occurs on the 

 shores of Ascension, which was formed by the dash of 

 the waves. Beautiful crystals of selenite were obtained by the writer in logs of 

 half decomposed wood in the shore cliffs near Callao, which were of similar origin. 



