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of the ferric oxide dissolves and a red coloured liquid is obtained. Similarly if ferric oxide is mixed with a large quantity of solid potash and a little potassium nitrate, the mixture is melted, kept molten for a time, cooled, and dissolved in water, a red coloured liquid is produced. These red liquids evolve oxygen, and precipitate ferric oxide, when heated. By measuring the oxygen evolved and determining the ferric oxide precipitated, conclusions can be drawn as to the composition of the compound in the original red solution. Such measurements have been made; they are in keeping with the hypothesis that the red liquids contain potassium ferrate, K,FeO. This salt appears to exist only in solution in much potash. The change which occurs when this solution is heated is probably as follows ;

2K,FeO2+ KOHAq + 2H2O = (x + 4)KOHAq + Fe ̧0, +30.

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Although no acidic oxide of iron has been isolated, yet we may say that the formation of potassium ferrate shews that a compound of iron with much oxygen would be an acid-forming oxide.

Chromium and manganese must then be classed both with 203 the metallic and with the non-metallic elements. A consideration of their physical properties alone would lead us to place them in the class metals; a consideration of the chemical properties of their lower oxides would confirm this conclusion; but a consideration of the chemical properties of their higher oxides shews that the elements in question are fairly closely related to the undoubtedly non-metallic elements sulphur and chlorine.

The properties of oxides are evidently conditioned by the 204 chemical characters of the elements with which oxygen is combined, and also by the ratio of the numbers of combining weights of oxygen and the other element which are united in a reacting weight of each oxide. The properties of hydroxides are also conditioned by the chemical characters, and by the relative masses, of the elements which are combined in a reacting weight of each hydroxide.

As we advance in our study of classes of elements and compounds we shall find that a similar statement holds good for each class of compounds.

Our examination of the properties and compositions of 205 classes of compounds has shewn that such terms as basic oxide or acidic oxide are relative. The oxide CrO, is acidic in its reactions with water and alkalis, but it is basic in its

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reactions with concentrated acids: the oxide MnO, is acidic in its reactions with alkalis, but not with water; it is basic in its reactions towards concentrated acids. It is only the oxides of the very positive elements which are basic, and the oxides of the very negative elements which are acidic, in all their reactions. But even in these cases, the terms basic and acidic imply that the interactions of the oxides with other compounds (acids and alkalis) have been examined.

Chemistry is not the study of elements and compounds alone, but it is the study of the interactions of elements and compounds.

The importance of chemical classification is so great that an examination of another group of elements will be made before we pass to other parts of our subject.

The elements nitrogen, phosphorus, arsenic, antimony, and bismuth are placed in the same class.

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Nitrogen and bismuth are the only elements of the class which occur uncombined with others in nature. Oxides and sulphides of the other elements, or compounds of these with other compounds, are found in several rocks; none of the elements except phosphorus and nitrogen occurs very widely distributed or in very large quantities. The elements arsenic, antimony, and bismuth are obtained by reducing their oxides by heating with finely powdered charcoal. Phosphorus is obtained by heating calcium phosphate (Ca2PO,) with charcoal.

These elements all combine directly with oxygen and the halogens, and all except nitrogen combine directly with

sulphur. Hydrides of all, except bismuth, are known. Several oxyacids of each element, except bismuth, are known. The oxides of bismuth are basic, one is feebly acidic; the oxides of arsenic and antimony are acidic, but also slightly basic; the oxides of nitrogen and phosphorus are acidic. Each element forms at least one oxychloride. Several saltsi.e. derivatives of acids obtained by replacing hydrogen of the acids by positive elements-of bismuth, and a few salts of arsenic and antimony, are known. Neither nitrogen nor phosphorus, nor any oxide of these elements, interacts with acids to form salts.

The compositions of the more important compounds of the 208 elements of the nitrogen group may be expressed by the following formulae, where M = N, P, As, Sb, or Bi.

Hydrides. MH, (none known when M = Bi).
Oxides. MO, MO, MO, MO, MO,.

Sulphides. M,S,, M ̧ ̧.

Haloid compounds. MX,, MX, (X = Cl, Br, I, F.). Oxyhaloid compounds. MOX, MOX, and many complex compounds M_OX.

Oxyacids. HMO,, HMO, HMO, HMO, &c.

Salts of commoner acids. M.6NO, M3S0, M3CO.: MONO,, MO(SO4),, &c.; chiefly known when M = Bi.

The hydrides, MH, when M = N, P, As, or Sb, are colour- 209

less gases at ordinary temperatures and pressures.

Hydride of arsenic or antimony is formed when hydrogen

is produced in a solution of a compound of arsenic or antimony; thus with zinc and dilute sulphuric acid,

{6Zn

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O,Aq}

́6Zn + 6H2SO2Aq + As,O,Aq = 2AsH, + 3H2O+6ZnSOAq xZn + xH2SO Aq = xZnSO ̧Aq+xH ̧.

Part of the hydrogen is evolved and part reacts with the

arsenious oxide.

Nitrogen hydride (ammonia) is generally prepared by heating solid salammoniac (ammonium chloride) with lime; thus

2NH,C1+CaO = CaCl, + H,O+2NH,.

Phosphorus hydride may be obtained by a similar process from phosphonium iodide; thus

PHI+ KOHAq = PH, + KIAq + H2O.

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The hydrides all dissolve in water; NH, and PH, combine with acids to form compounds similar in properties to the salts of the alkali metals; thus

MH2+ HCl = MH ̧CI; MH ̧+HI = MH1I &c. (M = N or P).

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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.

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,O,) 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 N,O,), 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.

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M = N, P, Sb, or Bi.

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When any one of the elements, except nitrogen, is heated in oxygen the oxide M2O, is formed; in the case of phosphorus, PO, and in the case of antimony, Sb.,, is also produced. Nitrogen and oxygen combine when electric sparks are passed through a mixture of the gases; N,O, and NO, are produced. The oxides M2O, are usually produced from MO, by an interaction between M,O, 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,O, and M,O are formed by reducing the higher oxides; the methods of reduction employed are very indirect. oxides MO, are changed to M,O,, by the direct action of oxygen. The oxides M,O,, when MN or Sb, are changed to MO by the direct action of oxygen; when M = P the oxide is easily changed to M,O, by the direct action of oxygen; when M = As

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or Bi the oxides are unchanged by heating in oxygen. oxides M2O,, except P,O,, are decomposed by heat, giving off oxygen, and forming either M,O, (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,O, cannot be classed either as distinctly acidic or basic.

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The oxides M,O,, except Bi,O,, are acidic; NO, and P,O, dissolve in water to form nitrous acid HNO,, and phosphorous acid H.PO,, respectively (N,O,+ H2O+ Aq = 2HNO2Aq; P ̧O2+ 3H ̧Ö + Åq = 2H,PO,Aq); arsenious acid (?H,ASO) has not been isolated, but salts, e.g. KASO,, are obtained by the reaction of AsO, with alkalis and basic oxides; antimonious acid HSbO, is known, although it is not obtained by the reaction of water with the oxide Sb2O... The oxide Bi2O is basic; it interacts with acids to form salts and water, thus Bi2O2+6HNO,Aq = 2Bi3NO2Aq+ 3H2O. Besides being acidic towards alkalis and the more distinctly basic oxides, the oxides As2O, and Sb,O, are basic towards many acids; thus each interacts with hydrochloric acid to form a chloride and water; M2O2+6HClAq = 2MC1,Aq + 3H2O. These oxides also appear to interact with concentrated sulphuric acid to form sulphates M,3SO,, but there is some doubt as to the compositions of the products of these reactions.

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The oxides M2O, are distinctly acidic, except Bi,O,. 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, (Bi̟,O) interacts with acids to form the same salts as are produced when BiO, 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 (Bi2O) in suspension, the properties of the substances obtained render

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