HYDROCHLORIC ACID.
The solution is placed in a retort provided with a receiver and distilled until the residual fluid assumes a pasty consistence: the operation is then discontinued. If hydrochloric acid be present in the materials under examination, the distillate will have an acid reaction, and, upon addition of solution of nitrate of silver, a white precipitate, which is easily soluble in ammonia but insoluble in nitric acid and in short possesses all the properties of chloride of silver, will be formed.
NITRIC ACID.
The distillate, obtained as in the preceding process, is neutralized by the addition of potassa or soda, and evaporated to dryness. The residue is mixed with copper filings, and introduced into a glass tube closed at one end and provided at the other with a cork through which a delivery-tube passes. Sulphuric acid is then added to the mixture, the cork inserted, the tube heated, and the evolved vapors conducted into a solution of protosulphate of iron. The latter solution acquires a brown coloration which, upon addition of sulphuric acid, changes to a violet, if nitric acid be present. Upon conducting the disengaged gas into a solution of narcotine, the latter acquires a beautiful red color.
Another portion of the residue should deflagrate when saturated with an alkali and projected upon live coals.
SULPHURIC ACID.
In order to detect this acid, the solution obtained by treating the organs with water is not distilled but is concentrated to one-sixth of its original volume, and then agitated with ether for about ten minutes. By this treatment the ether takes up the free sulphuric acid, but not the acid sulphates present. After ten minutes contact, the ether is decanted and allowed to spontaneously evaporate. Upon treating the residue, which contains the free sulphuric acid and fatty substances, with water, a solution containing only the sulphuric acid is obtained. Nitrate of baryta is then added to a portion of the fluid: in presence of sulphuric acid, a white precipitate, insoluble in acids, is produced. If this is heated on charcoal before the blow-pipe, a mass is formed, which, when moistened with hydrochloric acid and placed upon a clean silver coin, produces a black spot on the metal. Another portion of the solution is mixed with copper and the mixture evaporated in a tube closed at one end: sulphurous acid is evolved towards the end of the operation. This gas is detected by allowing it to pass over paper saturated with a mixture of iodic acid and starch; a blue coloration is produced which, owing to the transformation of the iodine set free into hydriodic acid, subsequently disappears. (We have never been able to effect the disengagement of sulphurous acid spoken of above when an exceedingly dilute sulphuric acid was used, even upon evaporating the mixture to dryness, notwithstanding Orfila's statement that the reaction occurs very readily.)
PHOSPHORIC ACID.
The aqueous solution is evaporated to dryness, the residue taken up with alcohol of 44° B., the fluid again evaporated, and the second residue dissolved in water. Upon adding acetate of lead to the solution, a white precipitate is produced if phosphoric acid be present. The precipitate is washed, suspended in water and a current of sulphuretted hydrogen passed through the mixture. If the fluid is then filtered, and the excess of sulphuretted hydrogen expelled from the filtrate by boiling, a liquid possessing the distinctive properties of a solution of phosphoric acid will be obtained. This should then be submitted to the following tests: Some pulverized charcoal is added to a portion of the solution, the mixture evaporated to dryness, and the residue obtained introduced into a Hessian crucible heated to redness: in presence of a considerable amount of the acid, free phosphorous is liberated and burns with a bright flame in the upper part of the crucible. In case this reaction fails to occur, other portions of the fluid are treated with a solution of a baryta salt, which causes a white precipitate, soluble in nitric acid; with an ammoniated solution of sulphate of magnesia, which throws down a crystalline white precipitate; and by boiling with molybdate of ammonia, acidulated with nitric acid, which produces a yellow precipitation, or at least a yellow coloration of the solution.