281 



it becomes first opake and silvery, then crepitates, enlarges, and 

 assumes a dxdl white colour ; after which the different laminse, 

 which are very numerous, separate from each other. If a larger 

 portion of the fibre is exposed to the flame, its outer coat is elevated 

 in flakes, and the whole substance splits longitudinally on one side 

 in such a manner as to exhibit a section of the various coats. 



Each of the fibres or spicula of the axis is enveloped in a thin 

 membrane connected with the bark, and which doubtless deposits 

 the very numerous concentric coats of which it is formed, as the bark 

 deposits the concentric coats of which the axes of Gorgonia, Coral- 

 Hum and Isis are formed. 



The number of fibres in the axis appears to increase as the coral 

 grows, as some are much thinner than the rest, while they all ex- 

 tend from the base to the apex of the coral. The part of the fibre 

 which is enclosed in the sponge does not appear to increase in thick- 

 ness so fast as the other part of the fibre, and it is not covered with 

 any bark. 



The late Dr. Prout and Mr. Pearsall have furnished me with the 

 following account of the chemicals constituent of these fibres : — 



" When this coralloid substance is exposed to heat it decrepitates 

 strongly ; and if the experiment be made in a tube and the heat 

 suddenly applied, it bursts to pieces with a sort of explosion, and 

 gives off a perceptible quantity of water, and yields a strong pecu- 

 liar fishy odour. Under the blowpipe it melts into a porous slag, 

 so light as to swim in water, and reminding one of the substance 

 called Tabasheer, the addition of a little alkali converts it into pel- 

 lucid glass. These experiments show that this substance consists 

 principally of hydrated silica. 



" W. Prout." 



" To J. E. Gray, Esq. 



" The Royal Institution, 

 Friday Evening, April 18th, 1832. 



"Dear Sir, — I have examined the peculiar organic formation 

 which you showed the structure of by the microscope when I had 

 the pleasure of seeing you last : indeed I had more than one speci- 

 men of marine produce, but the present had something of this ap- 

 pearance. 



" When heated in very small glass tubes, portions of these spines 

 decrepitated violently, and gave off much water, but no appearance 

 of ammonia, so that very little, or perhaps no trace of organic matter 

 can be expected to remain. 



" The acids (sulphuric, muriatic, and nitric) did not seem to affect 

 them even when pulverized and boiled together. 



"They certainly are not carbonate of lime. 



" I obtained very minute traces of sulphuric acid and lime ; but 

 so very inappreciable, except by the slightly turbid appearance when 

 viewed between the source of light, that, if the indications proceed 

 from the spines or not, they must be considered as accidental. 



