604 APPENDIX 



spots here and there, or it may be red outside, but quite black inside, or the reverse may be 

 the case, while occasionally pieces of coral are met with in which the core and the external 

 layer are black and the intermediate layer is red, so that a cross -section shows a central 

 black spot, surrounded by an inner red and an outer black ring. It is sometimes stated that 

 the original colour can be restored to coral which has turned black by allowing it to lie in 

 water and then exposing it to the sun. The investigations which ha\e been made do not, 

 however, tend to support this statement. 



The substance of coral consists for the most part of calcium carbonate, which, as v,e 

 have already seen, is probably present in the form of calcite. This is impregnated with a 

 small amount of organic material, the presence of which accounts for the fact that coral 

 becomes black when heated. The specific gravity of coral is very near that of calcite, much 

 nearer than to that of aragonite, the other crystallised modification of calcium carbonate. 

 The specific gravity of pure calcite is 2'72, while that of precious coral, irrespective of colour, 

 lies between 2'6 and 2'7 ; the values found by Canestrini, for example, are 2'6T1 and 2'68. 

 Precious coral is rather harder than calcite, being placed between 3 and 4 on Mohs' scale, 

 but rather nearer to 4, so that we may write H = 3f . This greater degree of hardness is no 

 doubt due to the admixture of foreign material. Coral is soft enough to be easily worked 

 with a knife or file, or turned on the lathe, but it is not hard enough to admit of a very 

 brilliant polish, the material depending for its beauty more upon its fine colour. 



The chemical composition of coral is given by the following analysis by Tischer : 



Red coral. Black coral. 



Water (Hfi) . 

 Carbon dioxide (COj) 

 Lime (CaO) . 

 Magnesia (MgO) . 

 Ferric oxide (Pe203) 

 Sulphuric anhydride (SO3) 

 Organic matter 

 Deficiency, &c. 



From this can be calculated the following constituents : 



Calcium carbonate (CaCOj) . 

 Magnesium carbonate (MgCO^) 

 Calcium sulphate ('CaS04) 

 Ferric oxide (FcjOj) 

 Organic matter .... 



Water (H^O) 



Phosphates, Silica, &c., and deficiency 



Red coral. Black coral. 



86-974 85-801 



6-804 6-770 



1-271 1-400 



1-720 0-800 



1-350 3-070 



0-550 0-600 



1-331 1-559 



100-000 lCO-000 



Earlier analyses give similar, though somewhat varying, results ; it is clear therefore 

 that, broadly speaking, coral consists of calcium carbonate, with a small amount of magne- 

 sium carbonate. The proportions in which these two constituents are present are not 

 always the same ; young coral contains only about 1 per cent, of magnesium carbonate, while 

 older material may contain as much as 38 per cent. The greater the amount of magne- 

 sium present, the harder does coral become, and the extent to which its hardness exceeds 

 that of calcite probably depends upon the proportion of magnesium carbonate present in 

 it. We learn also from the analyses quoted above that the differences in the composition of 



