Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/540

Rh 528 CHEMISTKY [COPPER, SILVER, precipitate on passing hydrogen sulphide through solutions cf mercuric salts ; it is not decomposed by hydrochloric acid; when heated it becomes red. Native cinnabar and ver milion consist of the red modification of mercuric oxide. No tendency to combine with hydrogen has been observed in the case of the metals of this group, but they have all been obtained in combination with hydrocarbon radicles. Their organo-metallic derivatives correspond in composi tion to zinc ethyl, Zn(C 2 H 5 ) 2, the attempt to convert rnercurous chloride into a corresponding organo-metallic compound by the action of zinc ethyl has been unsuccess ful, mercury ethyl, Hg(C 2 H 5 ) 2, being formed instead and mercury separated ; thus : Hg 2 Cl 2 + Zn(C 2 H 5 ) 2 = Hg(C 2 H 5 ) 2 + Hg + ZnCl 2. It would appear, in fact, that only mercurous compounds can exist in which mercury is associated with negative radicles. It has been shown that the density of the vapour of each of the metals cadmium and mercury is the half of its atomic weight, in other words, the gaseous molecules of these ele ments are monatomic(p. 471). As we are not acquainted with the density of any other metal in the state of gas, it is impos sible to say whether these metals are peculiar in this respect. On comparing the properties thus briefly sketched of the elements of this group, the general resemblance will be at once noticed, but it will be remarked also, that, although in respect of many properties the variations are gradational in the same degree as the atomic weights, certain properties are not continuous, but recur periodically. Thus glucinum, zinc, and mercury, the first, third, and fifth terms of the group, have many characteristics in common which are apparently wanting in the case of magnesium and cadmium, the second and fourth terms. This is evidenced especially by certain of their physical properties, by the solubility of their hydroxides in alkalies, by the formation of basic salts, and by the behaviour of their sulphides. The elements of this group are in many respects closely related to the metals of the previous group, of which they may be regarded as forming a sub-group, the formulae of their compounds being similar in most cases ; the general behaviour especially of magnesium and its compounds, it will be evident, is in close accordance with that of calcium and its compounds. But many im portant distinctions characterize the two groups. Thus the metals of the one group decompose cold water ; their oxides are powerfully basic, and readily combine with water ; their carbonates are extremely stable ; and their sulphates are insoluble or very slightly soluble in water, and do not readily form double salts. The metals of the other group, however, either are without action on water, or de compose it only when heated ; their oxides, although basic, exhibit little tendency to combine with water ; their car bonates are comparatively unstable ; and their sulphates are mostly very soluble in water, and readily form double salts. In the one group the positive character becomes more pronounced as the atomic weight increases, whereas in the other the positive character diminishes with increase of atomic weight. Glucinum, apparently, is much less closely related to magnesium than is the latter to zinc, a ad mercury much less closely to cadmium than cadmium to zinc ; but zinc and cadmium are more closely related than zinc and magnesium. These conclusions, which result chiefly from the comparison of what may be termed chemical properties, are entirely confirmed by Thomson s thermochemical investigation of the reactions involved in the formation of a number of compounds of the metals of this group, as will be evident from the following tables : Reaction. Units of heat developed or absorbed. R=Mg. R=Zn. R=Cd. Solid compounds R,0 85,430 82,680 97,210 181,660 15.630 -4,240 112,840 106,090 20,660 23,410 19,880 18,030 65, 680 93J240 158.290 1 3,010 2.540 1 96,250 89,500 23, 820 20,290 R, 0, H 2 148,960 217,320 151,010 R, 0,, H, R, Cl a R, 0. 2, S0 2 , 7H 2 Aqueous solutions RC1 2, Aq 35,920 RS0 4 + 7H 2 0,Aq R,C1 2, Aq 186,930 180,180 34,800 31,220 27,690 R,0,S0 3 Aq RO, S0 3 Aq R(OH) 2, SO s Aq R(OH) 2 , 2HClAq R(OH) 2, 2C 2 H 4 2 Aq Reaction. Units of heat developed or absorbed. Reaction. Units of heat developed or absorbed. Solid substances Hg,,. 42,200 Aqueous solutions Hg 2, 3NO,HAq 5,790 Hg, ... 30,660 Hg 2 ,0,3N0 3 HAq 47,990 Hg,, CL 82,550 HgCL, An -3,300 Hg 2, Br 2 68,290 HgBr 4 K 2 ,Aq ... -9,750 Hg,, L 44,440 HgCl 2 ,2KClAq... -1,380 H&amp;gt;, CL. 63,160 HgBr 2, 2KBrAq.. 1,640 Hg, Br. 50,550 HgI 2, 2KIAq 3,450 EC i t .: 34,310 Hg, C1 2, Aq 59,860 Hg,13r 4 ,K 2 ... 242,400 Hg0 2, 2HClAq... 18,920 COPPER SILVER G OLD. Name. Symbol. At. wt. Sp. gr. At. vol. Electric con ductivity. Copper Cu 63-3 8-9 7-1 96 -4 at 13 C. Silver Ag 107-6 10-5 10-2 100-0 ,, oc. Gold Au 196-2 19-3 10-1 77-9 These metals may be regarded as forming a sub-group to the metals of the alkalies. Copper is a very tenacious, ductile, malleable metal, of red colour, more easily fused than gold, but less fusible than silver. It is without action on water at a red heat, and does not oxidize in the air at ordinary temperatures, but when heated to redness it rapidly combines with oxygen forming a black oxide CuO ; when in a finely divided state, it readily unites with chlorine at ordinary temperatures. Moderately diluted nitric acid dissolves copper with great facility, and it is also dissolved by heated concentrated sulphuric acid ; hydrochloric acid, even when boiling, has little action on the massive metal, but slowly dissolves the finely divided metal. Copper forms two chlorides, cuprous chloride, CugClg, and cupric chloride, CuCl 2, but only one iodide Cu 2 I 2. Cuprous chloride is a white crystalline substance insoluble in water, but it dissolves in hydrochloric acid or ammonia, forming colourless solutions. Cupric chloride is readily soluble ; it is liver-coloured, but its solution in water is blue; it forms crystalline double salts with the chlorides of the alkali metals. It is converted into cuprous chloride and chlorine at a red heat. Cupric hydroxide, C^OH)^ separates as a pale blue precipitate on the addition of an alkali to a solution of a cupric salt ; in the dry state it is stable at 100 C., but at a slightly higher temperature it is converted into the black oxide. Cupric hydroxide, although insoluble in alkalies, 1 These numbers refer to a sulphate of the composition CdS0 4 + f H,0