10 Mix together 10 parts of prussian blue in powder, 5 parts of Class VI. the red oxide of mercury, and 30 parts of water, and boil the mixture for some minutes in a glass vessel. The blue colour disappears, and the mixture becomes yellowish green. Pour it upon a filter; and after all the liquid part has passed, pour parts of hot water through the filter to wash the residuum completely. The oxide of mercury decomposes prussian blue, separates its colouring matter, and forms with it a salt soluble in water. The liquid therefore which has passed through the filter contains the colouring matter combined with mercury, The other component parts of the prussian blue being insoluble, do not pass through the filter. Pour this mercurial liquid upon 24 parts of clean iron filings, quite free from rust. Add at the same time 1 part of concentrated sulphuric acid, and shake the mixture. The iron filings are dissolved, and the mercury for-. merly held in solution is precipitated in the metallic state. The cause of this sudden change is obvious: The iron deoxidizes the mercury, and is at the same instant dissolved by the sulphuric acid, which has a stronger affinity for it than the colouring matter has. There remains in solution therefore only sulphate of iron and the colouring matter. Now the colouring matter being volatile, which the sulphate of iron is not, it was easy to obtain it apart by distillation. Accordingly he distilled the mixture in a gentle heat the colouring matter came over by the time that one-fourth of the liquor had passed into the receiver. It was mixed, however, with a small quantity of sulphuric acid; from which he separated it by distilling a second time over a quantity of carbonate of lime. The sulphuric acid combines with the lime and remains behind, which the colouring matter cannot do, because carbonic acid has a stronger affinity for lime than it has. Thus he obtained the colouring matter in a state of purity. It remained now to discover its component parts. He formed a very pure prussian blue, which he distilled, and increased the fire till the vessel became red. The small quantity of water which he had put into the receiver contained a portion of the blue colouring matter and of ammonia; and the air of the receiver consisted of azote, carbonic acid gas, and the colouring matter. He concluded from this and other experiments, that the colouring matter is a compound of ammonia and oil. But when he attempted to verify this theory by combining together ammonia and oil, he could not succeed in forming colouring matter. This obliged him to change his opinion; and at last Chap 1. Of Berthollet, Proust, he concluded that the colouring matter is a compound of ammonia and charcoal. He mixed together equal quantities of pounded charcoal and potash, put the mixture into a crucible, and kept it red-hot for a quarter of an hour; he then added a quantity of sal ammoniac in small pieces, which he pushed to the bottom of the melted mixture, kept it in the fire for two minutes till it had ceased to give out vapours of ammonia, and then threw it into a quantity of water. The solution possessed all the properties of the prussian alkali. Thus Mr. Scheele succeeded in forming the colouring matter. This colouring matter was called prussic acid by Morveau in the first volume of the chemical part of the Encyclopedie Methodique; an appellation which soon became general in Great Britain and France. These experiments of Scheele were repeated and carried still farther by Berthollet in 1787; who applied to the explanation of the composition of the colouring matter the light which had resulted from his previous experiments on the component parts of ammonia. He ascertained, in the first place, that the phlogisticated alkali is a compound salt, containing prussic acid, the alkali, and oxide of iron; that it may be obtained in octahedral crystals; and that when mixed with sulphuric acid, and exposed to the light, it lets fall a precipitate of prussian blue. His next object was to ascertain the component parts of prussic acid. When chlorine is poured into prussic acid, obtained by Scheele's process, the acid becomes more odorous and more volatile, less capable of combining with alkalies, and precipitates iron from its solutions, not blue, but green. He conceived that in this case it had combined with oxygen, acquired new properties, and been converted into a new substance, which he called oxyprussic acid. From these experiments, and others similar to them, Berthollet concluded, that prussic acid does not contain ammonia ready formed; but that it is a triple compound of carbon, hydrogen, and azote, in proportions which he was not able to ascertain. This conclusion was verified by Mr. Clouet, who found, that when ammoniacal gas is made to pass through a red-hot porcelain tube containing charcoal, a quantity of prussic acid is formed.* This experiment does not succeed unless a pretty strong heat be applied to the tube.+ In the year 1806, a most valuable paper on prussic acid and * Ann. de Chim. xi. 30, Jour. de l'Ecole Polytechn. I. iii. 436. its compounds was published by M. Proust.* Sect. I. Though he did Class VI. not succeed in procuring the acid in a separate state, nor in ascertaining its constituents with rigid accuracy; he pointed out a great number of new and important facts which were of much utility, and which facilitated the subsequent investigation of Gay-Lussac. To this last illustrious chemist we are indebted Gay-Lussac, for an easy process for obtaining prussic acid in a state of purity,† for an accurate description of its properties when thus obtained, and for a rigid determination of its composition and constitution.‡ He showed that it was a combination of hydrogen, with a compound body, to which he gave the name of cyanogen. This body is analogous to the supporters, and capable like them of combining with different bases and converting them into acids. || Mr. Porrett had previously shown§ that it was capable of and Porrett. combining with sulphur, and forming a new acid, to which the name of hydro-sulpho-cyanic acid has been since given. And Berthollet had shown that it was also capable of uniting with chlorine, and forming an acid which has got the name of chlorocyanic acid; though the real nature of this acid was first ascertained by Gay-Lussac. Still considerable obscurity hung about the salt originally formed by Macquer, and called by him phlogisticated potash. It was afterwards denominated prussiate of potash, and its nature and properties were successively investigated by different chemists. But the nature of the precipitates which it forms with metallic salts remained unexamined till Berzelius published a set of experiments on the subject in the year 1819,¶ which settled several points with great accuracy; though there are still some others which admit of more than one explanation. Some additional facts have been more lately added to this interesting branch of chemical science by Wöhler, Gay-Lussac, Liebig, and Mr. Edmund Davy. I shall endeavour to lay as clear a view as possible of all the different branches of this difficult but interesting subject in the following sections. SECTION 1.-OF CYANOGEN. I have given an account of the method of preparing cyanogen, and described its properties in the first volume of this work (p. 209). Chap. I. Composition. It is a compound of two volumes of carbon vapour and one volume of azotic gas united together, and condensed into one volume. Hence we obtain its specific gravity, by adding together twice the specific gravity of carbon vapour, and the specific gravity of azotic gas. Sp. gr. of carbon vapour = 0·4166 × 2 = 0·8333 = 0·9722 1.8055 = specific gravity of cyanogen gas. Volumes of carbon and azotie gas bearing the same relation to each other that atoms do, it is obvious that cyanogen is a compound of We may obtain the specific Action of cy anogen on ammonia, So that its atomic weight is 3-25. gravity of the gas, by multiplying its atomic weight by 0.5555. Cyanogen has the property of combining with a great variety of bases, and of forming a set of saline compounds exceedingly analogous to the chlorides, and which have received the name of cyanodides. The greater number of these compounds possessing the properties of salts, I shall reserve the description of them till I come to the salts, which will occupy our attention in a subsequent part of this volume. But there are one or two compounds of a more complicated nature, which may be noticed here. 1. When a current of cyanogen gas is passed through an aqueous solution of ammonia, no fewer than four different compounds are formed. 1. Prussiate of ammonia, or hydrocyanate of ammonia, as it is also called. 2. The dark brown substance described in the first volume of this work (p. 211), as precipitated when a solution of cyanogen in alcohol is set aside. 3. Oxalate of ammonia. 4. A peculiar crystallizable body, resembling cyanodide of ammonia, but certainly a different substance." It is probable that the oxalic acid and hydrocyanic acid are produced by the mutual action of the water and cyanogen on each * Wöhler, Kong. Vetens. Acad. Handl. 1824, p. 328. other. For four atoms of cyanogen and three atoms of water, might, by mutually decomposing each other, produce one atom of oxalic acid and three atoms of hydrocyanic acid. There is a surplus of 1 atom of azote, which must either enter as a constituent into the dark brown matter or into the crystallized substance. This crystallized substance may be obtained pure by decomposing cyanodide of lead by caustic ammonia, or cyanodide of silver by sal ammoniac. It has a white colour, crystallizes in needles, and dissolves easily in water and in alcohol. Its solution is neutral, and it is not precipitated by lead or silver, or by any of the other metallic salts. When digested in caustic potash no ammonia is given out. It seems to contain water of crystallization. When heated to redness with potassium abundance of cyanodide of potassium is obtained. Class VI. ted hydrogen 2. If alcohol be saturated with cyanogen gas, and then with of sulphuretsulphuretted hydrogen gas, it assumes a dark yellow colour, and and cyanogen. soon after small red crystals are deposited. They have a fine scarlet colour, considerable lustre, and are opaque. They are scarcely soluble in cold water; but they dissolve in hot water, from which, however, they are again deposited as the liquid cools. This substance is but little soluble in alcohol. When gently heated it partly sublimes unaltered, but the greatest part becomes black, while much sulphuretted ammonia is disengaged, and a black coaly matter remains. When passed through hot oxide of copper a great deal of ammonia is disengaged, which does not render lime water muddy, and therefore contains no carbonic acid. Wöhler, to whom we are indebted for all these facts, ascertained by digesting it in aqua regia that it contained 53 per cent. of sulphur. I think it not improbable that these crystals consist of hydrosulphocyanic acid. This acid contains 54-23 per cent. of sulphur, which approaches pretty near the estimate of Wöhler. However, the action of it on the alkalies does not correspond with this notion of its nature. |