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Mr. Silliman, Jr. on the Composition of Corals. 191 
A few remarks are added upon some of their physical char- 
acters, before giving the mode and results of analysis. 
ardness.—All the various corals examined were superior in 
hardness to calcareous spar or common marble, and not inferior 
to Arragonite ; while some were as hard as apatite or crystallized 
phosphate of lime; or according to the scale used by mineralo- 
gists, the usual hardness will be expressed by 4, though occa- 
sionally as high as 5. Using an iron mortar in the earliest trials, 
the iron pestle was roughened and cut under the resistance of the 
- angular masses of coral, to a degree quite remarkable considering 
the nature of the substance operated on. So much iron was com- 
municated to the powder from this source, that recourse was had 
to a mortar of porcelain, and even this was not proof against wear, 
the porcelain pestle being pitted by the repeated blows. ‘I'he more 
porous species were crushed of course with less difficulty ; and 
this was especially the case with the species of Porites.* 
Specific gravity. —The specimens were reduced to fine pow- 
der before trying the specific gravity, as the porous character of 
the coral would otherwise interfere with obtaining correct results. 
Considerable variation will be observed in the following table. 
The numbers correspond to the catalogue on page 190. 
‘Specimens: | Sp: Grav] Specimens. |_Sp. Grav.) Specimens. | Sp: Grav. 
No. 1 | 2817 | No.20 | 2217 |No. 31 688 
2-732 2-564 33 | 2500 
4 | 2-564 23 | 2:353 34 | 2500 
2 | 21 25 | 2578 Meand. }) 9.571 
0 | 2105 26 | 258 pi 
| 12 | 2-497 27 | 2740 | Chama, ¢! 2897 
* On this subject Mr. Dana remarks, p. 711:—‘‘ The hardness of these coral 
secretions, which is much above that of common carbonate of lime, as stated by 
Mr. Silliman, is not fully explained by the peculiar chemical composition de- 
tected by this chemist. We suggest, as one cause, that the calcareous portion 
may have, in its intimate texture, the structure of Arragonite, or prismatic car- 
bonate of lime, instead of that of common rhombohedral cale spar. The Arra- 
gonite structure has been shown to be due to crystallization at a higher tem- 
perature than that which is required for cale spar, the two minerals being iden- 
tical in composition : in consequence of this higher temperature, a different erys- 
talline form is assumed; and, moreover, the material has a higher degree of hard- 
ness, that of Arragonite being designated by 34 to 4, while common calc spar or 
rhombohedral carbonate of lime is 3. These remarks, it will be perceived, bear 
upon the internal calcareous secretions of other animals. In connexion, it should 
be observed, however, that distinct rhombohedrons of calc spar have been de- 
tected by Mr. Carpenter in the shells of some Molluscs.” 
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