COMPOSITION OF CORALS. 



99 



on account of its elasticity and brittleness. The specific gravi- 

 ty 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 branch- 

 ing Madrepora and the massive Astrasas are good exam- 

 ples, consist almost wholly of carbon ate of lime, the same in- 

 gredient which constitutes ordinary limestone. In 100 parts, 

 95 to 98 parts are of this constituent ; of the remainder, there 

 are 1£ to 4 parts of organic matter, and some earthy ingredi- 

 ents amounting usually to less than 1 per cent. These earthy 

 ingredients are phosphate of lime, with sometimes a trace of 

 silica. A trace of fluorine also has been observed. 



S. P. Sharpies found the following constitution for the spe- 

 cies below named {Am. Jour. Sci., III., i. 168). 



Oculina arbuscula, N. Car. 

 Manicina areolata, Florida 

 Agaricia agaricites 

 Siderastraea radians 

 Madrepora cervicornis 

 Madrepora palmata 



CARBONATE 



PHOSPHATE 



WATER AND OR- 



OF LIME. 



OP LIME. 



GANIC MATTERS. 



. . . 95.37 . 



. 0.84 . 



. 3.79 



. . . 96.54 . 



. 0.50 . 



. 2.96 



. . . 97.73 . 



. 0.53 . 



. 1.64 



. . . 97.30 . 



. 0.28 . 



. 2.42 



. . . 98.07 . 



. 0.32 . 



. 1.93 



. . . 97.19 . 



. 0.78 . 



. 2.81 



Forchhammer found 2*1 per cent, of magnesia in Coral- 

 lium rubrum, and 6*36 in Isis hippuris. 



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

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

 tained. The same powers of elaboration which exist in other 

 animals belong to polyps ; for this function, as has been re- 

 marked, 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 chloride of calcium takes the 

 place of these. The powers of life may make from the ele- 



