Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/536

Rh 624 C H E M I S T 11 Y [METALS OF THE ALKALIES C(C 2 II 5 ) s .ONa ; and botli are acted upon by acetic chloride in a similar manner Si(C 2 H 5 ) 3 .OH + C,H 3 OC1 = Si(C 2 H 5 ) 3 Cl + C 2 H 4 O 2. C(C 2 H 5 ) 3 .OH + C 2 H 3 OC1 = C(C 2 H 5 ) 3 C1 + C 2 H 4 O 2. The most important difference is in their behaviour on oxidation, triethylsilicol remaining unattacked, while tri- ethylcarbinol is readily converted into acids containing fewer atoms of carbon. Silicon ethyl is converted by the action of chlorine into a chlorinated derivative Si(C 2 H 5 ) 3 (C 2 H 4 Cl), from which the silicon alcohol SiC s H 19. OH may be prepared, corresponding to the yet unknown carbon alcohol C 9 H 19 .OH. It is a liquid insoluble in water, smelling like camphor, and boiling at 190C By heating the compound Si(C 2 H 5 )(OC 2 H 6 ) 3 with hydriodic acid solution it is converted into siliconpropionic acid C,H 5 .Si(OC 2 H 5 ) 3 + SHI = C 2 H 5 .SiO(OH) + 30^,1 + H 9 O. In a similar manner, siliconacetic acid, CH 3 .SiO(OH)&amp;gt; may be prepared from the corresponding methyl compound. But although these silicon acids correspond in composi tion to acetic acid, CH 3 .CO(OH), and propionic acid, C 2 H 5 .CO(OH), they exhibit very different properties ; thus, they are white amorphous substances, insoluble in water, although soluble in alkaline solutions, from which they are precipitated by the addition of acids, whereas acetic and propionic acids are colourless liquids, soluble in water, and boil respectively at 119 C. and 140 C. When the vapour of carbon disulphide is passed over a heited mixture of silica and carbon, silicon disulphide, SiS 2 , is produced ; it crystallizes in white silky needles, which quickly decompose in moist air into hydrogen sulphide and amorphous silica. From the foregoing description of the silicon compounds, it will be evident that while closely allied both in composi tion and in many of their properties to the carbon com pounds, they nevertheless differ from them in numerous important particulars. Thus, carbon dioxide is gaseous, and silicon dioxide is a non-volatile solid ; the chlorides of carbon are stable in presence of water except perhaps at relatively very high temperatures, but the chlorides of silicon are with the greatest readiness decomposed by water ; carbon disulphide is a volatile liquid not affected by water, while silicon disulphide is a solid which cannot exist in .presence of water ; and obviously the representa tives of the carbon compounds oxalic acid, acetic acid, and propionic acid in the silicon series possess very different properties. In many respects silicon bears considerable resemblance to boron, the resemblance being especially noticeable between the elements themselves, and in the behaviour of their haloid compounds with water, and also in the property which the fluorides of both elements possess of combining with hydrogen fluoride. It is of interest to note that much more heat is developed in the formation of the oxides of boron and silicon than in the formation of carbon dioxide, which alone is gaseous ; thus (B 2, 3 ) = 317,200 units of heat. (C, 2 ) = 93,600 (Si, 2 )= 219,200 In the case of the corresponding chlorides the order of volatility is reversed ; thus BC1 3 CC1 4 SiCl 4 Boiling Point 17 78 50 Name. Symbol. At. wt. Sj&amp;gt;. gr. At. vol. Melting point. C. Electric conductivity at 20-21-5 C. Lithium Li 7 59 11-8 180 19-00 Sodium Na 23 97 237 97 37-43 Potassium K 39 86 45-3 62 20-83 Rubidium Rb 85-2 1-52 56 -C 58 Caesium Cs 1327 In discussing the remaining elements it will suffice to indicate the general nature of their relations to each other, as a full description of the more important will be given under other headings. It will be convenient in the first instance to consider those elements together which are most closely related in properties, and afterwards to indicate the manner in which the elements generally are related to each other. METALS OF THE ALKALIES. The elements of this class are white metals, volatile at high temperatures ; lithium is softer than lead but harder than sodium, while sodium is harder than potassium, and potassium harder than rubidium, the last mentioned being as soft as wax. They may all be separated from their chlorides by electrolysis, and apparently also by strongly heating mixtures of their carbonates with charcoal in iron retorts ; the latter method is employed in the manufacture of sodium and potassium, and rubidium has been prepared by it. Caesium has not yet been obtained in a pure state, but an amalgam of caesium may be procured l&amp;gt;y submitting its chloride to electrolysis, employing a globule of mercury as the negative electrode. Caesium is the most electro-positive element yet dis covered ; the remaining members of the group follow it in this respect in the order of their atomic weights. They are easily fusible (see table above), and their compounds with other elements are all fusible. The metals of this group and their compounds furnish characteristic spectra, which are distinguished from those of most other elements by their simplicity. Lithium and its salts communicate a beautiful red colour to flame, sodium salts an intense yellow, and potassium, caesium, and rubidium salts a violet colour. According to Troost and Hautefeuille, when potassium is heated to 350-400 C. in an atmosphere of hydrogen, it is converted into a hydride of the composition K H ; and the corresponding hydride, Na 2 H, may be prepared in a similar manner from sodium. Lithium, however, manifests but little tendency to combine with hydrogen, absorbing only 17 times its volume of the gas at 500 C. The hydrides are white bodies resembling silver in appearance; potassium hydride is very brittle, but sodium hydride is as soft as sodium, although it becomes brittle when heated ; the former takes fire spontaneously in air, but the latter is much more stable. The compounds of lithium, sodium, and potassium with hydrocarbon radicles are only known in combination with the zinc compounds ; thus, the body obtained by the action of sodium on zinc ethyl has the composition ZnNa(C 2 H 5 ) 3. The haloid compounds of the elements of this group may be formed by the direct combination of the metals with halogens ; their affinity for halogens, however, appears to be inversely proportional to their atomic weights. Thus, it is requisite to heat sodium to a moderately high tempera ture in an atmosphere of chlorine in order to secure its conversion into the chloride Nad ; but potassium inflames in chlorine at the ordinary temperature. Similarly, sodium may be preserved unchanged in contact with bromine, and is scarcely affected even when heated with it to 200 C.; potassium, however, causes a violent explosion when thrown on bromine. Sodium may also be fused with iodine with out appreciable reaction occurring, but potassium at once combines with it with explosive violence.
 * RS5* SUlcoBpropionlc acid.