ignite, as access of air is prevented by the water. But if air be carefully blown by the mouth through a long glass tube upon the bottom of the wine-glass, combustion will ensue, which is visible, especially in the dark.

Experiment 8.-Heat gently a piece of phosphorus of the size of a pea, placed in the middle of a glass tube, about Fig. 62.

twelve inches long. When ignition commences, remove the lamp. While the tube is held horizontally, the combustion is feeble and imperfect, because the heavy smoke, consisting of phosphoric and phosphorous anhydrides, passing off slowly, allows the admission of only a small quantity of air. But the combustion immediately becomes more vivid on inclining the tube, and when the tube is held perpendicularly it is complete, as then the draught of air is most powerful. In this way phosphorus may be oxidized to either degree required; it may be slowly burnt to form phosphorous anhydride, or completely, to form phosphoric anhydride.

Phosphorus, like sulphur, is capable of existing in several different states (allotropic states). Besides the ordinary kind the most interesting is that called red, or amorphous phosphorus. This is prepared by heating ordinary phosphorus to a temperature of about 240° C. (464° F.) for some time, in a vessel filled with carbonic anhydride to prevent it from burning. Red phosphorus is curiously different in properties from the ordinary kind, being insoluble in carbon disulphide, not poisonous, and only inflammable at a high temperature. It is now used extensively in the manufacture of "safety matches." The tips of these matches do not contain phosphorus, but when rubbed on the outside of the box they take up a little of the red phosphorus with which the side is covered, and combustion ensues.

PHOSPHINE, PH3.

Experiment 1.-Put into an ounce flask a quarter of an ounce of slaked lime, and a piece of phosphorus the size of a pea, fill it up to the neck with water, and place it in a small vessel containing a strong solution of salt, prepared by adding

half an ounce of common salt to an ounce and a half of water. Fit to the flask a bent glass tube, one end of which is made to dip into a basin of water; heat the salt water to boiling, and a gas will be evolved, which, as it issues from the tube Fig. 63.

and comes in contact with the air, takes fire spontaneously. This gas consists chiefly of phosphine, or phosphuretted hydrogen, but contains several combinations of phosphorus and hydrogen. If you collect it in a small jar filled with water, it immediately takes fire upon the admission of air. Both the phosphorus and the hydrogen combine with the oxygen of the air, and there results phosphoric anhydride, PO, and water, H2O. The first appears as a white smoke, which, when it issues in separate bubbles from the water, rises in rings.

Phosphuretted hydrogen, when unburnt, emits the smell of garlic.

In this experiment, the flask is placed in salt water, in order to guard against the ignition of the phosphorus, in case the flask should accidentally break. Salt water, at the strength specified, will not boil under 228° F. (109° C.); consequently, the boiling in the flask is more active than if it had been placed in pure water, the temperature of which, under ordinary pressure, can only be raised to 212° F. (100° C.). The apparatus for heating substances by means of hot water or saline solutions is called a water or saline bath. By such contrivances extracts are evaporated, and substances dried, which, at a stronger heat, would easily burn, or otherwise be decomposed.

Phosphine, when pure, is inflammable, but not spontaneously inflammable. When prepared as above it contains a little of the vapour of another compound of phosphorus and hydrogen

—a liquid—which has the formula P2H. It is the latter compound which is spontaneously inflammable.

It has been already stated that these oxides can be formed by the direct union of oxygen and phosphorus. The lower oxide and its corresponding acid are unimportant. Both oxides are white, snow-like powders, and they greedily combine with water to form acids.

Phosphoric Anhydride.

Experiment 1.--Ignite a small piece of phosphorus on a plate and cover it with a dry bottle or bell-jar. Phosphoric anhydride will be abundantly formed, as a white cloud, which settles on the sides of the bottle and on the plate. Shake the powder into a saucer, and pour a little water on it. A sharp hissing noise will be heard, testifying to the energy with which combination ensues. A solution of phosphoric acid is produced.

Phosphoric Acid, H3PO4.

Experiment 2.-Place a piece of phosphorus and a little water in a flask, and carefully heat them until the water boils; then add half an ounce of nitric acid and continue the heat. The phosphorus slowly dissolves, and is oxidized into phosphoric acid. Phosphoric acid prepared in this way is of course contaminated with the excess of nitric acid left undecomposed. This can be removed by evaporating to dryness and boiling the residue with water.

Phosphoric acid is tribasic, containing three replaceable atoms of hydrogen; and inasmuch as one, two, or all three of these atoms may be replaced by metals, its salts are very numerous. With sodium, for instance, the three following salts may be obtained:

NaH2PO4, Sodium di-hydrogen phosphate.
Na HPO, Di-sodium hydrogen phosphate.
Na PO, Tri-sodium phosphate.

A very great variety of compound salts may be formed by substituting part of the hydrogen with one metal, and part with another. As, for instance, in ammonium magnesium phosphate, NH,Mg"PO4, in which one atom of hydrogen is replaced by the monad group of atoms, or compound radical, NH, and the remaining two atoms by the diad metal magnesium, Mg".

Experiment 3.-Add ammonia to a solution of sodium phosphate, and then a little solution of magnesium sulphate, Mg"SO, (Epsom salts). A white precipitate will be produced, consisting of the above-mentioned ammonium magnesium phosphate :

MgSO4 + Na2HPO1 + NH2HO = Na2SO1+

Phosphoric acid or any other soluble phosphate would produce the same effect. Ammonium hydrate and magnesium sulphate therefore serve as a test for phosphoric acid and its salts. The body of an adult man contains

from 9 to 12 pounds of bones, containing

6 8 pounds of bone ashes, containing

5 7 pounds of calcium phosphate, containing

وو

1, 13 pounds of phosphorus.

Phosphates are also contained in the blood, flesh, and other portions of the body. Whence does it obtain this phosphorus? The answer is, from the meat and vegetables which it consumes. The phosphates occur in bread, in all kinds of grain, in leguminous and many other plants, particularly in their seeds. But how do the plants obtain these salts? By means of the soil. If arable land contained no such salts, no seeds could be produced. If we increase their quantity by mixing ground bones with the soil, we place the latter in a situation to produce a larger quantity of grain; consequently, bones furnish us with a powerful manure.

Besides ordinary phosphoric acid, two other compounds of phosphoric anhydride are known, differing in constitution from ordinary phosphoric acid by the relative quantities of water combined with the anhydride. These compounds are distinguished by the prefixes "pyro" and "meta" from ordinary or ortho-phosphoric acid. To form ortho-phosphoric acid three molecules of water are necessary, while for pyro

phosphoric acid two are required, and for meta-phosphoric

acid only one:

=

2H,PO, Ortho-phosphoric acid.

=

HP2O, Pyro-phosphoric acid.
P2O+ H2O = 2HPO3, Meta-phosphoric acid.

These three acids form salts which correspond to them in constitution. The common or ortho-phosphates contain the triad radical PO4, the pyro-phosphates, the tetrad radical P2O7, and the meta-phosphates, which resemble the nitrates in constitution, the monad radical, PO.

Experiment 4.-Heat common sodium phosphate to redness in a crucible for some time. Water is driven off, and sodium pyro-phosphate remains :

2Na2HPO1 = NaP2O, + H2O.

Sodium meta-phosphate may be prepared by heating microcosmic salt, Na(NH)HPO, (sodium ammonium hydrogen phosphate). Ammonia and water are expelled, and the meta-phosphate remains:

NaNHHPO4 = NaPO + NHg + H,O.

These pyro- and meta-phosphates may be dissolved in cold water without change, but if the solutions are boiled water is taken up and common phosphates obtained.

When common phosphoric acid is evaporated to dryness, meta-phosphoric, or "glacial" phosphoric acid is obtained. This, when boiled with water, is reconverted to ortho-phosphoric acid :

Experiment 5.-To solutions of sodium ortho-, pyro-, and meta-phosphates add a little solution of silver nitrate. With the ortho-phosphate, a yellow precipitate, and with the others white precipitates are produced. This test distinguishes the ortho-phosphate from the other two. To distinguish between these, add to each of the salts, first some acetic acid, and then a little white of egg beaten up with water. A white precipitate is produced with the meta-phosphate; none at all with the pyro-phosphate, or ortho-phosphate.