Prep. (Phosphorus, four hundred and thirteen grains; nitric acid, four fluid ounces; distilled water, twenty fluid ounces or a sufficiency. To the acid mixed with water, placed in a retort in a sand-bath, add the phosphorus; then apply heat until five fluid ounces have distilled over; put these again into the retort, and renew and continue the distillation until the phosphorus ha3 entirely dissolved. Transfer the contents of the retort to a porcelain capsule, and evaporate the liquid, raising the heat a little towards the close of the process, until bubbles of orange vapour cease to form, and a colourless liquid of a syrupy consistence is obtained. Lastly, add to the acid, when it lias cooled, as much distilled water as may be requisite to make it accurately measure a pint.) When nitric acid diluted with water acts with the aid of heat upon phosphorus in a glass retort, the acid is decomposed into oxygen, which unites with the phosphorus to form phosphoric acid, and nitric oxide gas which escapes; a portion of the nitric acid distils over, and is returned into the body of the retort; the liquid which remains in the retort when all the phosphorus is dissolved, is then evaporated to a small quantity, in order to drive off any undecom-posed nitric acid, and the syrupy solution of phosphoric acid afterwards diluted to the proper strength.

Prop. & Comp. As thus prepared, dilute phosphoric acid is a colourless liquid, without odour, of an agreeable acid taste; sp.

gr. 1.08. It contains between nine and ten per cent. of tribasic phosphoric acid in solution in water. Formula of acid, 3 HO, Po5. It docs not precipitate chloride of barium or nitrate of silver, nor is it coloured by sulphuretted hydrogen either before or after strips of silver or copper have been digested in it, these tests show the absence of sulphuric acid, chlorides, metallic impurities, or nitric acid. With ammonio-nitrate of silver phosphoric acid gives a canary-yellow precipitate soluble in ammonia, and in dilute nitric acid. When evaporated it leaves a residue which melts at a low red heat, and upon cooling exhibits a glassy appearance. It is not precipitated by a solution of albumen, which shows that it is not the monobasic variety of the acid. When mixed with an equal volume of pure sulphuric acid and then introduced into the solution of sulphate of iron, it does not communicate to it a dark colour, showing the absence of nitric acid.

Six fluid drachms poured upon 180 grains of litharge in fine powder, leave after evaporation a residue, which heated to dull redness weighs 215.5 grains, indicating the presence of 34.5 grains of dry acid (Po5). [Acidum Phosphoricum Dilutum, U. S., prepared by a process identical in principle to that in the text, has a sp. gr. of 1.056. It is likewise directed to be prepared by dissolving a troy ounce of glacial phosphoric acid in three fluid ounces of distilled water, adding 40 grs. of nitric acid, boiling until reduced to a syrupy liquid, free from the odour of nitric acid, and then adding distilled water to make the diluted acid measure 12 1/2 fluid ounces].

Therapeutics. Dilute phosphoric acid acts in a similar manner to dilute sulphuric acid, but is less powerfully astringent. It has been asserted to allay thirst in diabetes, and is supposed to exert an influence on the growth of osseous tumours.

Dose. 10 min. to 30 min. and upwards, freely diluted.

Adulteration. Sulphuric acid, hydrochloric acid, and metallic impurities detected by the above tests.