COMPOSITION OF CORALS. 99 
on account of its elasticity and brittleness. The specific gravi- 
ty of coral varies from 2°5 to 2°8: 2523 was the average from 
fifteen specimens examined by Prof. Silliman. 
Chemically, the common reef-corals, of which the branch- 
ing Madrepora and the massive Astreas are good exam- 
ples, consist almost wholly of carbonate 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 14 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. Sharples found the following constitution for the spe- 
cies below named (Am. Jour. Scz., IIIL., 1. 168). 
CARBONATE PHOSPHATE WATER AND OR- 
OF LIME. OF LIME. GANIC MATTERS. 
Ocalina arbuscula, N. Car.. . . 95.37 . . 0.84 . . . 38.79 
Manicina areolata, Florida. . . 96.54 . . 0.50 . . . 2.96 
Agaricia agaricites ieee eo Tao. |). 0.8) OLDS! a 20.5 | cee Mal 4 
Siderastrea radians Meme O30) cos, MOLLBe sree stn (eee 
Meaitopora cervicornis . /. . .- 98.07 . .. 0.82 . . = 1.93 
Madrepora palmata Mee aM OTTO et it OL 10h ty aif swans Ol 
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- 
