246 



Mr Silliman on the 



Specific Gravity. — The specimens were reduced to fine pow- 

 der before trying the specific gravity, as the porous charac- 

 ter of the coral would otherwise interfere with obtaining cor- 

 rect results. Considerable variation will be observed in the 

 following table. The numbers correspond to the catalogue 

 on page 244. 



The average from the sixteen species of corals is 2*523. 



Colour. — In general, the colour of the specimens examined 

 was white, or nearly so ; but some of them, as the Dendro- 

 phyllia nigrescens, and blue Heliopora (H. ccerulea), were 

 highly coloured. The colouring matter, in all cases, proved to 

 be organic, and was usually due to some trace of the animal 

 tissues. The highly coloured ones, when powdered, burnt 

 white, giving out, at a red heat, the odour of animal matter. 

 The Heliopora dissolved in chlorohydric acid, without having 

 its colour altered, and gave a light indigo-blue solution. A 

 drop of nitric acid, however, discharged this colour, and am- 

 monia threw it down as a brown precipitate. Heat imme- 

 diately destroys it. It is, therefore, evident that the colour- 

 ing matter is entirely organic, and is in no way connected 

 with the mineral constitution of the coral. However, some 



of common rhombohedral calc-spar. The arragonite structure has been 

 shewn to be due to crystallization at a higher temperature than that 

 which is required for calc-spar, the two minerals being identical in com- 

 position. In consequence of this higher temperature, a different crys- 

 talline form is assumed ; and, moreover, the material has a higher de- 

 gree of hardness, that of arragonite being designated by 2>^ to 4, while 

 common calc-spar, or rhombohedral carbonate of lime, is 3. These re- 

 marks, it will be perceived, bear upon the internal calcareous secretions 

 of other animals. In connexion, it should be observed, however, that 

 distinct rhombohedrons oi calc-spar have been detected by Mr Carpenter 

 in the shells of some Molluscs." 



