Page:EB1911 - Volume 06.djvu/314

Rh  Analogous bromine and iodine compounds are unknown, since bromides and iodides on heating with potassium bichromate and concentrated sulphuric acid give free bromine or free iodine.

The oxyfluoride, CrO2F2, is obtained in a similar manner to the oxychloride by using fluorspar in place of common salt. It may be condensed to a dark red liquid which is decomposed by moist air into chromic acid and chromic fluoride.

The semi-acid chloride, CrO2·Cl·OH, chlorochromic acid, is only known in the form of its salts, the chlorochromates.

Potassium chlorochromate, CrO2·Cl·OK, is produced when potassium bichromate is heated with concentrated hydrochloric acid and a little water, or from chromium oxychloride and saturated potassium chloride solution, when it separates as a red crystalline salt. By suspending it in ether and passing ammonia, potassium amidochromate, CrO2·NH2·OK, is obtained; on evaporating the ether solution, after it has stood for 24 hours, red prisms of the amidochromate separate; it is slowly decomposed by boiling water, and also by nitrous acid, with liberation of nitrogen.

Chromic sulphide, Cr2S3, results on heating chromium and sulphur or on strongly heating the trioxide in a current of sulphuretted hydrogen; it forms a dark green crystalline powder, and on ignition gives the sesquioxide.

Chromic sulphate, Cr2(SO4)3, is prepared by mixing the hydroxide with concentrated sulphuric acid and allowing the mixture to stand, a green solution is first formed which gradually changes to blue, and deposits violet-blue crystals, which are purified by dissolving in water and then precipitating with alcohol. It is soluble in cold water, giving a violet solution, which turns green on boiling. If the violet solution is allowed to evaporate slowly at ordinary temperatures the sulphate crystallizes out as Cr2(SO4)3·15H2O, but the green solution on evaporation leaves only an amorphous mass. Investigation has shown that the change is due to the splitting off of sulphuric acid during the process, and that green-coloured chrom-sulphuric acids are formed thus—

since, on adding barium chloride to the green solution, only one-third of the total sulphuric acid is precipitated as barium sulphate, whence it follows that only one-third of the original SO4 ions are present in the green solution. The green salt in aqueous solution, on standing, gradually passes back to the violet form. Several other complex chrom-sulphuric acids are known, e.g.

[Cr2(SO4)4]H2; [Cr2(SO4)5]H4; [Cr2(SO4)6]H6

(see A. Recoura, Annales de Chimie et de Physique, 1895 (7), 4, p. 505.)

Chromic sulphate combines with the sulphates of the alkali metals to form double sulphates, which correspond to the alums. Chrome alum, K2SO4·Cr2(SO4)3·24H2O, is best prepared by passing sulphur dioxide through a solution of potassium bichromate containing the calculated quantity of sulphuric acid,

K2Cr2O7 + 3SO2 + H2SO4 = H2O + K2SO4 + Cr2(SO4)3.

On evaporating the solution dark purple octahedra of the alum are obtained. It is easily soluble in warm water, the solution being of a dull blue tint, and is used in calico-printing, dyeing and tanning. Chromium ammonium sulphate, (NH4)2SO4·Cr2(SO4)3·24H2O, results on mixing equivalent quantities of chromic sulphate and ammonium sulphate in aqueous solution and allowing the mixture to crystallize. It forms red octahedra and is less soluble in water than the corresponding potassium compound. The salt CrClSO4·8H2O has been described. By passing ammonia over heated chromic chloride, the nitride, CrN, is formed as a brownish powder. By the action of concentrated sulphuric acid it is transformed into chromium ammonium sulphate.

The nitrate, Cr(NO3)3·9H2O, crystallizes in purple prisms and results on dissolving the hydroxide in nitric acid, its solution turns green on boiling. A phosphide, PCr, is known; it burns in oxygen forming the phosphate. By adding sodium phosphate to an excess of chrome alum the violet phosphate, CrPO4·6H2O, is precipitated; on heating to 100° C. it loses water and turns green. A green precipitate, perhaps CrPO4·3H2O, is obtained on adding an excess of sodium phosphate to chromic chloride solution.

Carbides of chromium are known; when the metal is heated in an electric furnace with excess of carbon, crystalline, C2Cr3, is formed; this scratches quartz and topaz, and the crystals are very resistant to the action of acids; CCr4 has also been described (H. Moissan, Comptes rendus, 1894, 119, p. 185).

Cyanogen compounds of chromium, analogous to those of iron, have been prepared; thus potassium chromocyanide, K4Cr(CN)6·2H2O, is formed from potassium cyanide and chromous acetate; on exposure to air it is converted into the chromicyanide, K3Cr(CN)6, which can also be prepared by adding chromic acetate solution to boiling potassium cyanide solution. Chromic thiocyanate, Cr(SCN)3, an amorphous deliquescent mass, is formed by dissolving the hydroxide in thiocyanic acid and drying over sulphuric acid. The double thiocyanate, Cr(SCN)3·3KCNS·4H2O, is also known.

Chromium salts readily combine with ammonia to form complex salts in which the ammonia molecule is in direct combination with the chromium atom. In many of these salts one finds that the elements of water are frequently found in combination with the metal, and further, that the ammonia molecule may be replaced by such other molecular groups as −NO2, &c. Of the types studied the following may be mentioned: the diammine chromium thiocyanates, M[Cr(NH3)2·(SCN)4], the chloraquotetrammine chromic salts, R¹2[Cr(NH3)4·H2O·Cl], the aquopentammine or roseo-chromium salts, R¹3[Cr(NH3)5·H2O], the chlorpentammine or purpureo-chromium salts, R¹2[Cr(NH3)5·Cl], the nitrito pentammine or xanthochromium salts, R¹2[NO2·(NH3)5·Cr], the luteo or hexammine chromium salts, R¹3[(NH3)6·Cr], and the rhodochromium salts: where R¹ = a monovalent acid radical and M = a monovalent basic radical. For the preparation and properties of these salts and a discussion on their constitution the papers of S. F. Jörgensen and of A. Werner in the Zeitschrift für anorganische Chemie from 1892 onwards should be consulted.

P. Pfeiffer (Berichte, 1904, 37, p. 4255) has shown that chromium salts of the type [Cr{C2H4(NH2)2}2X2]X exist in two stereo-isomeric forms, namely, the cis- and trans- forms, the dithiocyan-diethylene-diamine-chromium salts being the trans- salts. Their configuration was determined by their relationship to their oxalo-derivatives; the cis-dichloro chloride, [CrC2H4(NH2)2Cl2]Cl·H2O, compound with potassium oxalate gave a carmine red crystalline complex salt, [Cr{C2H4(NH2)2}C2O4][CrC2H4(NH2)2·(C2O4)2]1½H2O, while from the trans-chloride a red complex salt is obtained containing the unaltered trans-dichloro group [CrC2H4(NH2)2·Cl2].

 CHROMOSPHERE (from Gr. , colour, and  , a sphere), in astronomy, the red-coloured envelope of the sun, outside of the photosphere. It can be seen with the eye at the beginning or ending of a total eclipse of the sun, and with a suitable spectroscope at any time under favourable conditions. (See and .)

 CHRONICLE (from Gr. , time). The historical works written in the middle ages are variously designated by the terms “histories,” “annals,” or “chronicles”; it is difficult, however, to give an exact definition of each of these terms, since they do not correspond to determinate classes of writings. The definitions proposed by A. Giry (in La Grande Encyclopédie), by Ch. V. Langlois (in the Manuel de bibliographie historique), and by E. Bernheim (in the Lehrbuch der historischen Methode), are manifestly insufficient. Perhaps the most reasonable is that propounded by H. F. Delaborde at the École des Chartes, that chronicles are accounts of a universal character, while annals relate either to a locality, or to a religious community, or even to a whole people, but without attempting to treat of all periods or all peoples. The primitive type, he says, was furnished by Eusebius of Caesarea, who wrote (c. 303) a chronicle in Greek, which was soon translated into Latin and frequently recopied throughout the middle ages; in the form of synoptic and synchronistic tables it embraced the history of the world, both Jewish and Christian, since the Creation. This ingenious opinion, however, is only partially exact, for it is certain that the medieval authors or scribes were not conscious of any well-marked distinction between annals and chronicles; indeed, they often apparently employed the terms indiscriminately.

Whether or not a distinction can be made, chronicles and (q.v.) have points of great similarity. Chronicles are accounts generally of an impersonal character, and often anonymous, composed in varying proportions of passages reproduced textually from sources which the chronicler is seldom at pains to indicate, and of personal recollections the veracity of which remains to be determined. Some of them are written with so little intelligence and spirit that one is led to regard the work of composition as a piece of drudgery imposed on the clergy and monks by their superiors. To distinguish what is original from what is borrowed, to separate fact from falsehood, and to establish the value of each piece of evidence, are in such circumstances a difficult undertaking, and one which has exercised the sagacity of scholars, especially since the 17th century. The work, moreover, is immense, by reason of the enormous number of medieval chronicles, both Christian and Mahommedan.

The Christian chronicles were first written in the two learned languages, Greek and Latin. At an early stage we have proof of the employment of national languages, the most famous instances being found at the two extremities of Europe, the (q.v.), the most ancient form of which goes back to the 10th century, and the so-called Chronicle of Nestor, in Palaeo-Slavonic, written in the 11th and 12th centuries.