Take of phosphorus, one ounce; nitric acid, four fluid ounces; distilled water, ten fluid ounces. Add the phosphorus to the nitric acid, mixed with water, in a glass retort placed in a sand-bath; then apply heat until eight fluid ounces are produced. Put these again into the retort, that eight ounces may distil, which are to be rejected. Evaporate the remaining fluid in a platina capsule until it is reduced to two ounces and six drachms. Lastly, to the acid, when it is cold, add as much distilled water as may be sufficient to make up accurately twenty-eight fluid ounces by measure.

In this process the nitric acid is decomposed by the affinity of the phosphorus for oxygen being greater than that of the nitrogen. The necessity of diluting the acid is obvious; for when phosphorus is added to strong nitric acid, rapid combustion, attended with explosion, takes place; whereas in the diluted acid the phosphorus slowly attracts the oxygen, and binoxide of nitrogen is evolved. The repetition of the process is necessary to secure the acidification of the whole of the phosphorus: but even then a minute portion of it remains unacted upon.

Qualities. - Phosphoric acid obtained according to the above process is colourless, inodorous, and has a sharp acid taste; but it does not act on animal matter, like sulphuric and nitric acids. It forms permanent salts with alkalies and metallic oxides. When evaporated to dryness, and heated to redness, it forms meta-phosphoric acid. It consists of 2 eq. of phosphorus, 31.4 + 5 of oxygen =40, making the equivalent =71.4. Phosphoric acid forms insoluble precipitates with lime-water and the hydrochlorate of lime; baryta, magnesia, the salts of lead, and nitrate of silver: hence these substances are incompatible in prescriptions with it.

1 Annals of Philosophy, vol. v. p. 99.

Medical properties and uses. - Diluted phosphoric acid possesses tonic properties, and may be* beneficially administered in all cases in which the mineral acids are indicated. It is also indicated in cases in which the phosphates are deposited from the urine, and in exostosis and other bony tumours. The dose is from x. to xx., in water or any bland fluid.