CORALS AND CORAL REEFS — VATJGHAN. 205 



of experiments on the species common in the Tortugas, Florida, to 

 ascertain how long they can live out of the water and found that 

 all can withstand limited exposure in the air, but, of course, none of 

 them can live permanently out of water. Colonies of the same spe- 

 cies were placed in both the sun and shade on glass plates; and in 

 both the sun and shade in vessels containing enough sea-water to 

 keep the bases of the colonies wet. The death of colonies exposed to 

 the air naturally depends on the rate of the desiccation of the soft 

 animal tissues. As heat accelerates drying, the specimens in the 

 sun are more quickly killed than those in the shade ; and, as both 

 the soft parts and the skeletons of all corals are more or less porous, 

 a colony whose base is immersed in sea water will live longer than 

 one lying on a glass plate. Although not precisely, almost gener- 

 ally, those corals with the most porous skeletons can longest endure 

 being out of the water, for such skeletons dry more slowly than those 

 that are more compact, and, if the bases are wet, they rapidly ab- 

 sorb water through capillarity. Any one of the sixteen species of 

 Tortugas corals used in the experiments will endure half an hour's 

 exposure on a glass plate in the shade without apparent damage; 

 nearly all will stand one hour's exposure under such conditions; 

 while some survived such exposure for four hours. Colonies of a 

 number of species were badly damaged but were not entirely killed 

 after lying for one and a half hours on a glass plate in the sun. 

 Of the species experimented with Favia fragum, Porites porites, 

 and Porites astreoides have the greatest capacity for withstanding 

 exposure in the atmosphere, while that of Maeandra areolata and 

 Siderastrea radians is almost as great. Usually the species that form 

 the exposed reefs can not withstand being out of the water so long 

 as those that live on the shallow flats behind the reefs. Doctor 

 Mayer made a series of exposure experiments on the corals at Mur- 

 ray Island, Australia, and obtained similar results. 



RELATION OF CORALS TO CONCENTRATION OF SALTS IN THE OCEAN. 



The following is Dittmar's mean of 77 analyses of sea water : 



Cl 55. 292 



Br .188 



SO* 7. 692 



COa - • 207 



Na 30.593 



K 1.106 



Rb 



Ca 1.197 



Mg 3.725 



Fe, Si0 2 , P0 4 



Fe, NHi, NO3 



A1 2 3 , Fe 2 3) Si0 2 



100. 00 



