98 CORALS AND CORAL ISLANDS. 
plete restoration to their former size and powers. The frag- 
ment broken off, dropping in a favorable place, would become 
the germ of another coral plant, its base cementing by means 
of new coral secretions to the rock on which it might rest; or, 
if still in contact with any part of the parent tree, it would be 
reunited and continue to grow as before. The coral zodphyte 
may be levelled by transported masses swept over it by the 
waves; yet, like the trodden sod, it sprouts again, and contin- 
ues to grow and flourish as before. The sod, however, 
has roots which are still unhurt; while the zodphyte, which 
may be dead at base, has a root—a source or centre of life—in 
every polyp that blossoms over its surface. Hach animal 
might live and grow if separated from the rest, and would ul- 
timately produce a mature zodphyte. 
V. COMPOSITION OF CORAL. 
Ordinary corals have a hardness a little above that of com- 
mon limestone or marble. The ringing sound given, when cor- 
al is struck with a hammer, indicates this superior hardness. 
It is possible that it may be owing to the carbonate of lime be- 
ing in the state of aragonite, whose hardness exceeds a little 
that of ordinary carbonate of lime or calcite. It is a common 
error of old date to suppose that coral when first removed from 
the water is soft, and afterward 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 waters with 
which it is penetrated 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. A branched 
Madrepore rings on being struck when first collected ; and a 
blow in any part puts in hazard every branch throughout it, 
