& 
Mr. Silliman, Jr. on the Composition of Corals. 195 
potassa, alumina and iron, crystallize out, while any lime pre- 
viously combined is separated as sulphate ; and in the solution we 
must look for the phosphoric acid and magnesia, together with a 
little persalt of iron, held up by the alcohol. 
F. The mixture (E) being filtered, and the precipitate washed 
quite clean with alcohol, the filtrate is evaporated until ‘all the 
alcohol is expelled, and then supersaturated with ammonia; a 
little trace of alumina and iron separates, which may be added to 
that to be obtained from the other portion (H). We may now 
either add an excess of pure chloride of calcium to the filtrate 
(F'), or a portion of perchloride of iron. The object in either 
case is to separate the phosphoric acid in combination with a 
base, from whose weight its quantity may be directly estimated, 
which is an indispensable step, since the fluorine, according to 
this plan of analysis, can be estimated only by the amount re- 
quired to saturate the excess of bases. In case the chloride of 
calcium is employed, we have all the phosphoric acid in the form 
of phosphate of lime, mixed with a large quantity of sulphate of 
lime, derived from the sulphuric acid and sulphate of potassa 
previously employed. This mixture of phosphate and sulphate 
of lime is collected, washed, and redissolved in chlorohydric acid. 
The sulphate of lime is separated by alcohol, and the phosphoric 
acid remains in solution, which, after the excess of alcohol has 
been expelled, may be thrown down by ammonia, ignited and 
weighed, or preferably, may be estimated by a magnesian salt. 
If we employ the method by perchloride of iron, we form in the 
acid solution containing the phosphoric acid, a basic perphosphate 
of iron, on supersaturating the solution with ammonia. ‘This 
compound is mixed with a bulky mass of peroxide of iron, which 
being thrown on a filter and thoroughly washed, is subsequently 
decomposed completely by hydrosulphuret of ammonia, into 
sulphuret of iron and phosphate of ammonia. Care must be 
taken to use a sufficient quantity of perchloride of iron, other- 
wise a white precipitate of neutral perphosphate of iron is formed, 
which is soluble in'an excess of ammonia. In either case (the 
employment of the chloride of calcium, or the perchloride of 
iron) the phosphoric acid eliminated may be finally best estimated 
by a magnesian salt and ammonia, as the ammonio-phosphate of 
magnesia, from whose known constitution the phosphoric acid is 
easily calculated. We have employed both of these methods ; 
but on many accounts prefer that by the perchloride of iron. 
