PERIODIC RELATIONS.] CHEMISTRY 543 Two oxides of didymium corresponding to the two oxides of cerium are known. The lower oxide, Di 2 3 , is white, and in contact with water is slowly converted into the hydroxide, Di 2 (OH) 6 ; didymium hydroxide resembles aluminium hydroxide in appearance, but has a pale rose colour, and the salts formed by dissolving it in acids have either a pure rose or violet colour. By gently heating the oxide Di 2 O 3 in air it is converted into the higher oxide Di 2 4 , which has a dark brown colour ; it dissolves in hydrochloric acid with evolution of chlorine, and in oxy- acids with evolution of oxygen, forming the same salts as are produced on dissolving the lower oxide. Yttrium and erbium each form only one oxide, and like lanthanum, cerium, and didymium only one corresponding chloride. Yttrium oxide, Y 2 O b , is white, and erbium oxide, Er 2 3 , has a pale rose colour ; the former is readily soluble in acids, even after ignition, but the latter dissolves with great difficulty. The yttrium salts are colourless ; but the salts of erbium have a beautiful rose colour, and their solutions give an absorption spectrum, which is not the case with yttrium salts. When erbium oxide is strongly heated it glows with an intense green light, which in the spectroscope exhibits a continuous spectrum intersected by a number of bright bands corresponding in position to the dark bands in the absorption spectrum of solutions of erbium salts. Didymium oxide behaves similarly, and solutions of didymium salts give an absorption spectrum by which they are readily distinguished from erbium salts. The chlorides, nitrates, and sulphates of the cerite and gadolinite metals are readily soluble in water, but their carbonates are insoluble ; the solutions of their salts all possess a sweet astringent taste. The most conclusive evidence that lanthanum, cerium, didymium, and yttrium are closely allied to the alkaline earth metals is afforded by thermochemical investigation, the amounts of heat developed on neutralizing their hydroxides being for hydrochloric acid, according to Thomsen s experiments, inferior only to that developed on neutralizing the hydr oxides of the alkali and alkaline earth metals, as shown by the following examples : Name of Hydroxide. Units of he on neutra Sulphuric acid. at developed lizing with H/drochlorie acid. Difference. Hydroxides of alkali and alka line earth metals 1 31,300 27,470 26,030 25,720 25,070 26,480 24,920 23,820 16,100 27,600 25,020 24,160 23,980 23,570 22,950 21,390 20,290 13,640 3,700 2,450 1,870 1,740 1,500 3,530 3,530 3,530 2,460 Lanthanum hydroxide . Cerium ,, .... Didymium ,, Yttrium ,, .... Manganous , , Fe r rous ,, .. .... Cadmium , , Glucinum ,, Metallic thorium has been obtained as a dark-grey powder of the specific gravity 7 65 to 7 79, easily soluble in nitric acid, but difficultly soluble in hydrochloric acid. Its oxide, Th. 2 O 3 (if Th = 178 5), is white, and after igni tion is insoluble in all acids except concentrated sulphuric acid ; it appears to be destitute of acid properties as it does not expel carbon dioxide when fused with alkaline car bonates. Thorium hydroxide is precipitated from solutions of thorium salts by alkalies as a gelatinous mass, soluble in most acids, but insoluble in alkalies. Thorium chloride, Th 2 Cl 6 , is a white crystalline substance, which dissolves in water with rise of temperature. Thorium sulphate, Th 2 (SO 4 ) 3 , is crystalline, and, like the sulphates of the cerite and gadolinite metals, is more soluble in cold than in hot water. PERIODIC RELATIONS OF THE ELEMENTS. The foregoing description of the elements and of some of their more important compounds will be sufficient to show that, while each element manifests certain characters which distinguish it from all others, many of the elements are more or less closely related in properties, as indeed we have already frequently had occasion to point out. Elements which exhibit similar properties often differ in atomic weight to the same or nearly the same extent ; for example, the difference between the atomic weights of potassium and rubidium is about 46, and of rubidium and caesium 47 5, since K = 39, Rb = 85 2, and Cs=132 7. As these three elements are closely related in properties, rubidium differing from potassium to about the same extent that caesium differs from rubidium, we are led to suspect a connection between the atomic weight of an element and its properties, especially as with very few exceptions the elements all possess different atomic weights. Many instances of relation between atomic weight and properties, similar in character to that which obtains in the case of potassium, rubidium, and caesium, have been commented on from time to time by various chemists, but the connec tion between the atomic weights of the elements generally and their properties was not recognized until Mendeljeff in 1869 pointed out that the latter are periodic functions of the former. In other words, if the elements are grouped in the order of their atomic weights, it will be found that nearly the same properties recur periodically throughout the entire series. Hence the whole of the elements maybe arranged in a number of groups, each group consisting of members of the same natural family following each other in the same order. The elements are arranged in this manner in the following table, although, in order to retain elements which are undoubtedly members of the same natural family in the same vertical series, a few departures from the order of atomic weights are necessary, but probably they are necessary merely because the atomic weights are incorrectly determined ; thus, tellurium is placed before iodine, and osmium, iridium, and platinum before gold. If the position assigned to uranium be correct, the number at present accepted as its atomic weight is much too low. M M M H M ,{ -: 8.-J
- 1
M H i Li 7-0 Na 23-0 K 39 Cu 63-8 Rb 85-2 Ag 107-7 Cs 132-7 Mg 24 Ca 40 Zn t>5 Sr 87-2 Cd 111-6 Ba 136-8 G 9 3 B 110 Al 27-3 C 12 Si 28 Ti 48 Zv 90 Sn 117-8 N 14-0 P 31 V 51-2 As 75 Nb 94 Sb 122
16 S 32 Cr 52-4 Se 79 Mo 95-8 Te 128-1 F 19-1 Cl 35-4 Mn 55 Br 79-8 I 12G-5 Fe 56 Rn 103-5 Co 58-6 Rh 104 Ni 58-6 Pd 106 rt ?80-5 La 139 Ei- Co 141 Di 147 Ga ? In 113-4 Th ? 178-5 Ta 182 Bi 207-5 W 184 U ?180 Os 198-C Ir 196-7 Pt 196-7 L 196-2 Hff 199-8 mOS Tl 203-6 Pb 206-4 The position of hydrogen at the head of a series of metals is in accordance with Graham s conclusion that this element possesses the characters of a metal, a conclusion which many chemists are inclined to accept on account of the striking analogies to the metals which it exhibits in its relation to the halogens, oxygen, &c. As the properties of alloys are usually similar to those of their constituent metals, whereas the compounds of metals with non-metallic elements are in most cases widely different from those of the elements which enter into their composition, the fact that Troost and Hautefeuille s sodium and potassium hydrides (^sodium-hydrogen and potassium-hydrogen alloys)
(p. 524) retain the metallic character of sodium and potassium