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mass of that compound composed of definite numbers of smaller parts (combining weights) of two or more elements, which smaller parts are arranged in some definite way relatively to each other. The reactions of ammonium compounds, for instance, almost oblige us to think of the reacting weights of these compounds as each composed of one combining weight of nitrogen closely united, in some way, with 4 c.ws. of hydrogen, and this group of combining weights as less closely united with the other elements which form a part of the reacting weight of

the compound. 213 Let us return to the consideration of the properties of the

hydrides MH. These compounds may all be oxidised : phosphorus hydride is very easily changed to phosphorus pentoxide (PO) and water by mixing with oxygen and heating; arsenic and antimony hydrides are oxidised to oxide of arsenic or antimony (M,0,) and water, by burning in contact with a large quantity of oxygen ; ammonia is oxidised with difficulty, it is necessary to mix ammonia with a large quantity of oxygen and raise the temperature considerably, the products are water, nitrogen oxides (especially NO and 1,03), and

nitrogen. 214

The oxides of the nitrogen group of elements are numerous; the following table presents the compositions of the best studied of these oxides.

M.O.
M=N.

M = Ñ or Bi. M=N, P, As, Sb, or Bi.
MO

MO
M=N, P, Sb, or Bi. M=N, P, As, Sb, or Bi.

When any one of the elements, except nitrogen, is heated in oxygen the oxide M,0, is formed ; in the case of phosphorus, P,05, and in the case of antimony, Sb, 0,, is also produced. 'Nitrogen and oxygen combine when electric sparks are passed through a mixture of the gases; N,Oand N, O are produced. The oxides M,0are usually produced from M,0by an interaction between M,0g and some compound (e.g. nitric acid), or compounds (e.g. caustic potash solution and chlorine), from which oxygen is produced. The lower oxides M,0, and M,0 are formed by reducing the higher oxides; the methods of reduction employed are very indirect. The oxides M,O, are changed to M,O,, by the direct action of oxygen. The oxides M,0g, when MEN or Sb, are changed to M,O by the direct action of oxygen; when M = P the oxide is easily changed to M,0; by the direct action of oxygen; when M = As

M.O

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or Bi the oxides are unchanged by heating in oxygen. The oxides M,0, except P,0, are decomposed by heat, giving off oxygen, and forming either M,0, (M=N or Sb), or M,O, (M = As or Bi).

Nitrous oxide (NO) dissolves in water without forming an acid; but as the oxide can be obtained by heating an aqueous solution of hyponitrous acid (HNOAq), the oxide may be regarded as the anhydride of this acid.

The oxides M,0, cannot be classed either as distinctly acidic or basic.

The oxides M,0, except Bi, Oj, are acidic ; N.0, and P,0g dissolve in water to form nitrous acid HNO,, and phosphorous acid H PO, respectively (N,02 + H,0+ Aq = 2HNO Aq; P,0, + 3H,O + q=2H PO Aq); arsenious acid (?H_AsO ) has not been isolated, but salts, e.g. K AsOz, are obtained by the reaction of As,O, with alkalis and basic oxides ; antimonious acid H Sbo, is known, although it is not obtained by the reaction of water with the oxide Sb,Os. The oxide Bio, is basic; it interacts with acids to form salts and water, thus Bi 0,+6HNO, Aq=2Bi3NO, Aq+ 3H,0. Besides being acidic towards alkalis and the more distinctly basic oxides, the oxides As,O, and Sb,0, are basic towards many acids; thus each interacts with hydrochloric acid to form a chloride and water; M,O, + 6HCIAq=2MCI, Aq + 3H,0. These oxides also appear to interact with concentrated sulphuric acid to form sulphates M 380, but there is some doubt as to the compositions of the products of these reactions.

The oxides M.O, are distinctly acidic, except Bios. When M=N or P, the oxides dissolve in water to form the corresponding acids; the acids corresponding to the oxides when M= As or Sb are not obtained directly from these oxides by interaction with water. Bismuthic oxide, or bismuth pentoxide, (Bi,05) interacts with acids to form the same salts as are produced when Bi O, interacts, and oxygen is simultaneously evolved; bismuthic oxide is therefore a peroxide. From what we have learned concerning the properties of oxides, and from considering the properties of the highest oxides of the elements classed with bismuth, we might expect bismuthic oxide to exhibit some acidic functions. No salts are obtained by interactions between this oxide and alkalis ; but when the oxide in question is prepared by passing chlorine into very concentrated potash solution holding bismuthous oxide (Bi, 0) in suspension, the properties of the substances obtained render

it very probable that unstable compounds of bismuthic and potassium oxides are formed at certain stages of the preparation. The composition of these unstable salts may be expressed by the formula «Bi,OnYK,0; w and y probably vary according to the relative masses of Bi, O, and KOH used in the preparation of Bi,Os, and according to the temperature, &c. If bismuthic oxide exhibits any acidic functions they are certainly ex

tremely feeble. 215 The following table presents the compositions of the more

important oxyacids of the members of the nitrogen group; to each oxyacid there generally corresponds a certain oxide; that is, the oxide is obtained from the acid by removing hydrogen and oxygen (generally by heating the acid), or the acid is produced by the interaction of water and the oxide.

ŞOxide. N,O. N,Oz. N2O5.
Nitrogen

Oxyacid. HNO Aq.* HNO,Aq.* HNO3.
Oxide.

P,0g.

P,05.
Phosphorus
Oxyacid. H3PO, H,PO3. HPO.H,POH,P,0,.

As,Oz
Arsenic

(Oxyacid. salts known of forms MgAso, and MASO, (M=K, &c.)
Oxide.

As Oy
LOxyacid. HASO,.H.As.. H.As,0,.
Oxide.

Sb,0g.
Antimony

H2SbO2 0.xyacid.

salts of form MSb0, are also known. Oxide.

Sb,0, loxyacid. Hsbog. H,Sbo,.H.Sb,07. It will be noticed that the pentoxides of phosphorus, arsenic, and antimony, are represented as being each the anhydride of three acids.

Phosphorus pentoxide interacts with water to produce one or other of the three acids according to the relative masses of water and oxide used, and the temperature. The following equations represent the reactions ;

(1) P,O, +HO (cold) = 2HPO,;
(2) PO +2,0 (cold) =H.PO,;
(3) PO, + xH 0 (warm) = 2H PO, + (x – 3) H,O.

* The symbol Aq is here used to signify that the acids after which it is placed are known only in aqueous solutions.

(Oxide.

When the acid H,PO, is heated to about 200°, it loses water and forms the acid H,P,O,, when this is heated to about 400° it loses water and forms the acid HPO,; thus, (1) 2H PO, -4,0=H,P,0,; (2) H.P.0,-H,O=2HPOZ.

None of the acids corresponding to arsenic or antimony pentoxide is obtained by the interaction of the oxide with water. The acid H, Aso, is formed by oxidising arsenious oxide in presence of water (thus As,O, Aq + 51,0 + 401 = 2H, Aso, Aq + 4HCIAq); when this acid is heated it yields H.As, On, and then HASO. The acid H Sbo, is produced by the interaction of antimonic chloride with a little cold water; thus SbC1, +4H,0 = H SbO, +5HCl; the acid H Sb.O, is obtained by heating H_Sbô, to about 100', and the acid HS60, by heating H.$6,0, to about 200°.

Solutions of either ÅPO, or HP.0, when heated give a solution of H PO,; solutions of HÅsb, and H.As.0, even at the ordinary temperature change rapidly to a solution of H,AsO,; but solutions of HSbo, and H,Sb,0, seem to be more stable than a solution of H.SbO,

To the acids corresponding to the oxides M,0, are given 216 names ending in -ic; phosphoric, arsenic, antimonic, acid. The prefix ortho- is employed to distinguish the acid of the form H,MO,, the acid of the form HMO, is called meta-, and the remaining acid, H.M.O,, is called pyro. The acids, especially the phosphoric acids, are also distinguished as tribasic phosphoric acid H,PO, monobasic HPO,, and tetrabasic phosphoric acid H.P.O. Nitrogen forms a meta-acid only.

All the oxyacids of nitrogen are more or less easily de- 217 composed, by heat, or by reactions with other substances; hyponitrous acid (HNOAq), and nitrous acid (HNO,Aq), combine with oxygen at ordinary temperatures to form nitric acid (HNO3); these acids therefore act as reducing agents. Nitric acid we know is an energetic oxidising agent. The lower acids of phosphorus, H,PO, and H PO3, are reducing agents; but they do not combine with oxygen so rapidly or at such low temperatures as the lower acids of nitrogen do. An aqueous solution of arsenious oxide may possibly contain the acid H AsOy; this solution, like that of antimonious oxide, is a weak reducing agent. The highest acids of phosphorus can scarcely be classed as oxidising agents; they are fully oxidised, but they do not easily part with oxygen : these acids are not separated into oxide and water by heat alone. The highest oxides of arsenic and antimony (M,0z) are reduced by heat

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to lower oxides (M,0,) and oxygen ; these oxides therefore sometimes react as oxidising agents : inasmuch as the highest acids of arsenic and antimony can be separated by heat alone into oxide (M,0z) and water, it follows that these acids will sometimes react as oxidisers.

The preceding sketch of the oxides and oxyacids of the elements of the nitrogen groups shews how closely related these elements are to each other; but it also shews a gradation of properties from nitrogen to bismuth. Bismuth is evidently more widely separated from the other members of the group than these are from each other. Nitrogen and phosphorus are distinctly non-metallic, negative, elements; bismuth is metallic; arsenic and antimony stand midway between the metals and the non-metals; these elements are sometimes classed, with one or two others, as metalloids.

Our examination of some metals and non-metals shewed that non-metals sometimes exhibit allotropy. We might reasonably expect to find nitrogen and phosphorus existing each in more than one form, and it would certainly be incumbent on us to inquire whether arsenic and antimony exhibit allotropy or not. We should scarcely expect to find more than a single form of bismuth.

The existence of more than one form of nitrogen has not been proved; experimental results are however on record which point to the possibility of nitrogen undergoing allotropic change.

At least two distinct modifications of phosphorus are known. Ordinary phosphorus is a yellowish-white, semitransparent, crystalline, solid ; spec. gravity = 1.8; melting point = 44°; easily soluble in carbon disulphide, ether, and various oils. It combines with oxygen, the halogens, and sulphur, very rapidly and at low temperatures. It is extremely poisonous. When ordinary phosphorus is heated to about 240° in an atmosphere of carbon dioxide it is changed into a red amorphous solid. This change is more quickly and completely accomplished by heating ordinary phosphorus in a closed vessel to about 300°: a portion of the phosphorus is oxidised, and the rest is transformed into red phosphorus. Red, or amorphous, phosphorus is heavier than ordinary phosphorus; spec. gravity = 2.1; it is insoluble in carbon disulphide and ether; it combines with oxygen, the halogens, and sulphur only at fairly high temperatures. Red phosphorus is not poisonous. When heated in a stream of carbon dioxide

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