Chemical Composition of the Calcareous Corals. 247 



corals have a slight ferruginous tint, from the presence of a 

 little peroxide of iron, which will be seen to be an almost 

 constant constituent, although in exceedingly small quantity. 



Behaviour with re-agents. — All corals are rapidly dissolved 

 in dilute chlorohydric, nitric, or acetic acids, with brisk effer- 

 vescence and escape of carbonic acid. The solution is fre- 

 quently coloured by organic matter, which sometimes renders 

 it turbid. When the powdered coral is treated with pure 

 water, more or less of common salt, and other soluble saline 

 matters, derived from the evaporation of sea-water, are 

 washed out ; and this precaution was found necessary to in- 

 sure accurate results. 



The solution of a coral in nitric acid is very soon blackened 

 by a solution of nitrate of silver, from the presence of organic 

 matter. Ammonia, added to a solution in nitric or chloro- 

 hydric acid, with the least possible excess of acid, will gene- 

 rally produce an immediate precipitate of granular ammo- 

 nio-phosphate of magnesia, thus indicating the presence of 

 both magnesia and phosphoric acid. 



Chloride of barium produces, with a chlorohydric solution, 

 a granular, white precipitate, which is nearly all redissolved 

 in an excess of chlorohydric acid. (A small portion of sul- 

 phate of barytes is generally formed in using this test, owing 

 to the almost constant presence of a small quantity of sul- 

 phate of lime in the corals.) 



A portion, dissolved in nitric acid, and carefully neutralized, 

 when treated with nitrate of silver, will, on standing, deposit a 

 considerable yellowish precipitate of phosphate of silver, 

 which is redissolved in ammonia and nitric acid. 

 ' Acetate of lead, added to a chlorohydric solution, produces 

 a copious precipitate of chloride of lead, which is not wholly 

 redissolved by an excess of acetic acid, but is taken up by 

 nitric acid. These facts are a sufficient proof of the presence 

 of phosphoric acid. 



Lime-water, added to a solution of coral, either neutral or 

 slightly acid, will produce an immediate gelatinous precipi- 

 tate of all the bases and acids which the coral can contain, 

 except, of course, the lime and solvent acid. Great care is 

 needed in this operation to prevent the formation of a car- 



