Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/532

Rh 520 CHEMISTRY BORON. Symbol, B; At. vt, 11 ; Valency, &quot;. This element always occurs in the combined state as boric acid, or as a salt of boric acid. Two modifications of boron may be obtained, viz., an amorphous and a crystalline modification ; the former is produced by reduc ing boric anhydride, B 2 O 3, by heating it with sodium, and the latter is formed when the reduction is effected by the aid of aluminium at a very high temperature. Amorphous boron is a dark brown powder; it does not oxidize in the air at ordinary temperatures, but when heated it burns in air or oxygen, in the latter with dazzling brightness, forming the oxide B. 2 O 3. By ignition in an atmosphere of nitrogen it is converted into a white amor phous boron nitride, BN It does not decompose water, even at the boiling heat, but readily dissolves in nitric acid, producing boric acid ; when heated with potassium hydroxide it forms a potassium borate, hydrogen being evolved. On heating amorphous boron with aluminium it is dis solved, and crystallizes from the fused metal on cooling ; it may be separated from the latter by the aid of hydrochloric acid. The crystals are usually of a more or less browu colour, but in. lustre, refracting power, and hardness they are nearly equal to the diamond. Crystalline boron is only slightly oxidized at the temperature at which diamond burns. Boron forms a trichloride, BC1 3, a tribromide, BBr 3 , and a trifluoride, BF 3. The chloride may be obtained by the direct action of chlorine on amorphous boron, the com bination taking place at ordinary temperatures ; it is also produced by strongly heating a mixture of boric anhydride and charcoal in an atmosphere of chlorine, and by heating boric anhydride with phosphorus pentachloride to 150 C. The bromide may be obtained in a similar manner by the direct combination of its elements. Both are colourless, mobile liquids; the chloride boils at 17 C., and the bromide at 90 C. They are readily decomposed by water ; thus 3HC1. BC1 3 + 3H 2 = H 3 B0 Boron chloride. 33 Boric acid. Boron chloride is not easily deprived of its chlorine by the action of metals, which is apparently accounted for by the fact that much heat is evolved in its formation from its elements. According to Troost and Hautefeuille, the amounts of heat disengaged in the formation of the chloride and oxide of boron from their elements, and in the decomposition of the former by a large excess of water, are as follows (B, C1 3 ) = 104,000 units of heat. (B a ,0 g ) =317,200 (BCl 3 ,A&amp;lt;i) = 79,200 ,, Boron fluoride is formed by the action of hydrofluoric acid on boric anhydride, viz., by heating a mixture of boric anhydride and calcium fluoride with concentrated sulphuric acid. It is a colourless gas, of pungent, suffocat ing odour ; when passed into water, which dissolves about 700 times its volume of the gas, it is partly decomposed into boric and hydrofluoric acids, and partly combines with the hydrofluoric acid thus produced, forming fluoboric acid, HBF 4. This acid can only be obtained in a state of dilute solution, but many salts formed from it are known, such as potassium borottuoride, KBF 4, for example. Its existence appears to indicate that boron is capable of acting as a pentad element. Boron oxide, B 2 O 3, is most readily obtained by strongly heating boric acid; it forms a colourless, brittle, glassy mass, which dissolves readily in water, producing boric acid, and is one of the most stable oxides known. It unites with metallic oxides, when fused with them, forming borates, and at high temperatures it is capable of decom posing carbonates, sulphates, and indeed the salts of all acids the anhydrides of which are more volatile than itself. Boric acid, H 3 BO 3 or B(OH) 3, occurs native in many volcanic districts, especially in Tuscany, where it issues from the earth together with vapour of water ; borax, Na 2 B 4 O 7 + 10H 2 0, and a few other of its salts are also found in nature. It crystallizes from water in white nacreous laminae, which are easily soluble ; when heated to about 100 C., it furnishes a residue of the composition H B 4 O 9, which, on heating to 160 C., becomes H 2 B 4 O 7 , aud at a stronger heat it is converted into the anhydride, B 2 O 3. A large number of borates are known, but the nature of their relation to boric acid is not well understood. A comparatively small number are derived from the acid H 3 B0 3, the majority apparently being formed from an acid of the empirical composition HBO 2, to which the name metaboric acid is given. Salts derived from the acids H B 4 O (J and H 2 B 4 O 7 are also known, and ordinary borax, the most important of the borates, may be regarded as formed from the latter acid. CARBON. Symbol, C; At. wt., 11 97 ; Valency, iT . The properties of carbon itself and its oxides have already been described (p. 86), and need not therefore be again discussed. On comparing the compounds of carbon with those of other elements we find that, whereas there is reason to believe that the number of atoms of any of the other elements directly associated together in a molecule of their compounds is very small, probably never exceeding five, carbon compounds frequently contain a relatively very large number of carbon atoms, which from the behaviour of the compounds we are led to suppose are in direct association with each other ; and while none of the remain ing elements are known to furnish more than a single stable compound with hydrogen, the number of stable compounds of carbon with hydrogen which have been obtained may be counted by hundreds. We are thus led to the conclusion that carbon possesses two distinctive pro perties : firstly, that of uniting with itself to an almost unlimited extent in comparison with other elements ; and secondly, that of combining with hydrogen in numerous proportions. It is in consequence capable of uniting with the same elements in a multiplicity of proportions, and of furnishing a great variety of compounds, which probably exceed in number those of all the remaining, elements together. The study of the compounds of carbon constitutes that branch of our science which is termed organic chemistry, which is treated separately below, pp, 544 sqq. ; the study of the remaining elements and of their compounds constitut ing inorganic or, as it is also termed, mineral chemistry. The division is both useful and necessary, on account of the great number of carbon compounds which exist, and because, generally speaking, the compounds of other ele ments are distinguished from those of carbon by their com parative simplicity, and also by the comparative want of stability which all but the simplest exhibit ; this we may attribute to the possession by carbon of the distinctive pro perties above mentioned, but otherwise there is no essen tial difference between the compounds of carbon and those of the remaining elements. Organic chemistry originally dealt only with substances more or less directly derived from the animal or vegetable kingdom, and it was long believed that the chemist was