254
ELEMENTS OF MATERIA MEDICA.
other acids by the following characters: it throws down, with the solublesalts of lime, lead, and baryta, white precipitates (phosphates) soluble innitric acid: with the nitrate of silver it causes a yellow precipitate(subsesquiphosphate of silver) soluble in nitric acid and in ammonia:hydrosulphuric acid causes no change of colour or precipitate with it: ifthe soluble phosphate be heated to redness, it is converted into a pyro-phosphate, which gives, with the nitrate of silver, a white precipitate(pyrophosphate of silver).
Purity. —Diluted phosphoric acid, prepared according to the Pharma-copoeia , generally, if not invariably, contains traces of nitric acid. Sul-phuric acid may be recognized by chloride of barium, which produceswith it a white precipitate insoluble in nitric acid. Hydrochloric acid isdetected by nitrate of silver, which causes a white precipitate also inso-luble in nitric acid, but soluble in ammonia. The presence of metallicmatter may be known by hydrosulphuric acid. Saturated with carbonateof soda, no phosphate insoluble in water should be thrown down.
Composition. —Pure anhydrous phosphoric acid is thus composed:—
Eq. Eq.Wt. Per Cent. Berzelius . Dulong. Davy.
Phosphorus.1 .... 16 ... . 44*44 .... 44*05 .... 44*8 .... 42*6
Oxygen.... 20 ... . 55*56 .... 55*95 .... 55*2 .... 57*4
Phosphoric Acid . . 1 .... 36 ... . 100*00 .... 100*00 .... 100*0 .... 100*0
Mr. Phillips says, that 100 grains of the dilute acid of the Pharmacopoeia saturate 42 grains of [crystallized] carbonate of soda. If the whole ofthe free acid were phosphoric, the composition of the dilute acid wouldbe as follows:—
Phosphoric Acid. 10*5
Water. 89*5
Dilute Phosphoric Acid.100*0
Physiological Effects, (a.) On vegetables .—This acid is poisonousto plants (Gceppert, quoted by Decandolle, Phys. Veget.)
(b) On animals —-Very few experiments have hitherto been made withit on animals. Orfila ( Toxicolog. Gener.) found that a strong solution ofit acted like sulphuric acid. Thrown into the veins of a dog, it coagu-lated the blood and killed the animal within ten minutes. Introducedinto the stomach, it acted as a powerfully corrosive poison.
(c.) On man .—Diluted phosphoric acid produces the usual effects, ofthe diluted mineral acids, and which I have before noticed (pp. 80, 84,and 96). It is, therefore, denominated tonic and refrigerant, and may beemployed wherever the mineral acids are indicated. It is milder, moreassimilable, and, therefore, less likely to disagree with the digestiveorgans than sulphuric acid, with which, in its action, it is usually com-pared. These qualities it perhaps derives from its being, as Burdaich(Arzneimittellehre, Bd. 3, S. 395, 1809) expresses it, “ less heterojge-neous to the human organism, since it has a considerable share in ithecomposition of it.” The same authority also observes, that besides ful-filling the indications of the mineral acids, “ it much exalts the excitabi-lity when the organism is weak.”