Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/542

530 CHEMISTRY [THALLIUM. crystalline salt of the composition KAuO. 2 , 3H.,0, separating from the solution on concentration. On digesting auric hydroxide with ammonia it is converted into the so-called fulminating gold, which is obtained in the form of a yellowish-brown powder easily exploded by percussion or heat ; it is somewhat uncertain what is the composition of this body, but it is not improbable that it is represented by the formula An (NH 2 ) < QJLT . The distinctions between cjpper, silver, anc! gold are obviously very marked ; yet they resemble each other in many respects, the relation being especially evident when the cuprous and aurous compounds are compared with the argentic compounds. It is to be noted that their hydroxides and many other of their compounds are acted on by ammonia in a characteristic manner, both silver and gold hydroxides being converted into highly explosive bodies ; the nature of the products has not been ascertained with certainty, but apparently they are allied in composition. The ammoniacal derivatives of copper correspond in com position to those formed from nickel, a metal which in many respects is allied to copper. As in the magnesium group of metals, the positive character diminishes with increase of atomic weight, but to a much greater extent. Silver is the only element of the group which exhibits marked analogy with the metals of the alkalies, being connected with them by the isomorphism of its sulphate with that of sodium, by the formation of an alum, by the alkalinity of its hydroxide, and by its colour. Theriuo- chemical investigation also indicates a marked analogy between silver and the metals of the alkalies, the sulphates of silver, sodium, and potassium dissolving in water with absorption of heat, whereas nearly as much heat is developed on dissolving coppe-r sulphate as when the equivalent amount of sulphuric acid is addsd to water, copper sulphate resembling in this respect the sulphates of magnesium and zinc : thus : Ag. 2 S0 4 , Aq K. 2 S0 4 , Aq Na. 2 S0 4 , Aq CuSO 4 , Aq ZnS0 4 , Aq - 4,480 units of heat. -6,380 -60 16,300 18,500 20,300 The existence of double salts such as CuK 2 (SO 4 ) 9 , 6H 2 O, isomorphous with those derived from the sulphates of magnesium and zinc, and the readiness with which basic salts of copper are formed, are confirmatory of the con clusion that copper is allied to zinc and magnesium. The results of the thermo-chemical investigation of copper, silver, and gold are of considerable interest, as will be evident from the following tables. To illustrate the application of the values in these tables, it may be pointed out that the stability of the oxides of copper and the instability of silver oxide and of auric hydroxide are explained by the fact that much heat is developed in the formation of the oxides of copper from their elements, whereas the formation of silver oxide would involve but a slight development of heat, and the formation of auric hydroxide would even involve a large expenditure of energy. Then it will be noticed that, in the formation of cuprous iodide, heat is developed to the extent of 32,520 units per molecule, and that the formation of an aqueous solution of cupric iodide would involve the development of only 10,410 units per molecule ; hence it follows that the conversion of two molecules of cupric iodide into a molecule of cuprous iodide and a molecule of iodine would be attended by the development of no less than 32520 - 2 x 10410 = 11700 units of heat. This explains the fact that a mixture of cuprous iodide and iodine, and not cupric iodide, is obtained on adding potassium iodide to a solution of cupric sulphate: 2CuS0 4 + 4KI = Cu 2 I 2 + 1 2 + 2K 2 SO 4 . Again, the superior affinity of auric bromide for hydrogen bromide as compared with that of auric chloride for hydrogen chloride is shown by the development of 7700 units of heat in the formation of the compound AuBr 4 H, and of only 4530 units in the formation of the corresponding chlorine com pound AuCl 4 H ; and that the latter is almost completely decomposed and converted into the former on treatment with hydrobromic acid is evidenced by the development of 13,800 units of heat in the reaction, complete decomposition requiring the development of 14,300 units. Reaction. Units of heat developed. Reaction. Units of heat developed. Cu, 37,160 Cu 2 , 40,810 Ago. .., 5 900 Cu, C1 2 51,630 CuCl 2 , Aq 11,080 Cu, (J1 2 , Aq 62,710 Cu , Br 2 , Aq 40,830 Cu, I 2 , Aq 10,410 Cu 2) Clj 65,750 A", Cl 29,380 Cu 2 , Br 2 49,970 A" , Br 22,700 Cu- 2 , I., 32,520 Ag, I 13,800 Cu 2 0, 2FIC1 49,300 Af.,O, 2HC1 ... 77,220 Cu 2 0, 2HBr 60,640 A<*,0, 2HBr ... 90,980 Cu 2 0, 2HI 72,150 AgoO, 2III 102,140 Cu 2 0, 2HC lAq 14,660 Aff,0 , 2HClAq . 42,580 Cu./), 2HBrAq 20,760 Ag 2 0, 2HBrAq ... 51,100 Cu 2 0, 2IIIAq 33,730 Ag 2 0, 2HlAq 63,720 CuO, 2HClAq 35,270 Cu(OH) 2 , 2HClAq ... CuO, N 2 5 Aq 14,910 15,250 Aff.,0 , N,0,Aq .. 10,880 Cu(OH) 2 , N 2 5 Aq ... CuO, S0 3 Aq Cu(OH) 2 , S0 3 Aq 14,890 18,800 18,440 Ag 2 0,S0 3 Aq 14,490 Reaction. Units of heat developed or absorbed. Reaction. Units of heat developed or absorbed. Au, C1 3 22,820 AuCl 4 HAq, 4HBrAq 13 800 Au, Br 3 8,850 AuBr 4 HAq,4HClAq - 510 Au, Cl 5,810 AuCl 3 , Aq 4,450 Au, Br -80 AuBr 3 , Aq -3,760 Au, I -5,520 AuBr 4 H + 5H 2 , Aq -11,400 Au 2 , 3 , 3H 2 -13,190 Au , C1 3 , Aq 27,270 Au(OH) 3 , 3HClAq 18,440 Au , Br 3 , Aq 5,090 Au(OH) 3 ,3HBrAq AuCl.jAq, HClAq... AuBr 3 Aq, HBrAq... 29,180 4,530 7,700 Au , C1 3 , HClAq . Au,Br 3 , HBrAq ... 31,800 12,790 The values in this table have reference to the modification of gold obtained by reducing a solution of auric chloride with sulphurous acid. THALLIUM LEAD. Name. Symbol. At. wt. Sp.gr. At. vol. Electric conduc tivity at C. Thallium Lead Tl Pb 203-6 206-4 11-8 11-4 17-2 18-1 9-36 8-32 Thallium is a heavy crystalline metal, resembling cadmium in appearance ; it is extremely soft, even lead scratching it readily. It melts at 294 C., and boils below a white heat ; the liquid metal can hardly be distinguished from mercury. It is reduced from solutions of its salts by zinc, but not by cadmium. It readily dissolves in concen trated nitric acid, and is also dissolved by dilute sulphuric acid, especially if it be in contact with a piece of platinum, but it is only slowly attacked even by boiling hydrochloric acid ; it decomposes water at a red heat. It combines

with the halogens at ordinary temperatures. When exposed