133 CARBON. 204. Describe some of the principal forms of carbon. Jan. 1863. 1. Diamond is the purest representative of carbon. Its density is 3.5. It is the hardest body known. It crystallises in forms belonging to the cubic system, and is found native in the alluvial soils of India and Brazil. It is much valued as a gem on account of its hardness and transparency, and high refractive power for light. It is used to cut glass. Burned in oxygen it produces carbonic dioxide. 2. Graphite, or plumbago, which is often, but improperly, called black lead, is much used in the manufacture of pencils. Like diamond it is found native in various parts of the world, crystallised in forms belonging to the hexagonal system, also in masses. It is very soft, black with a metallic lustre, a good conductor of heat and electricity. 3. Charcoal is of two kinds-wood charcoal, originating from the distillation of wood in iron retorts; and animal charcoal, originating from the distillation of bones, also in iron retorts. Both are black, and are remarkable for the property of absorbing enormous quantities of gases, and of removing colour from liquids which owe their tint to the presence of organic matters. This property, however, is chiefly remarkable in the animal charcoal. Thus it is that wood charcoal is chiefly used for purifying and aërating water, and the animal charcoal for bleaching and disinfecting purposes. 4. Coke is the residue left in the retorts used for making coal gas. It is porous, hard, and possessed of a slight metallic sound. It is used for purposes of heating furnaces, and has the advantage of giving great heat and no smoke. 5. Soot is condensed smoke, deposited in chimneys. It is used as a manure on account of the ammonia it contains. 6. Lampblack is obtained by burning bodies rich in carbon, such as impure turpentine, in a limited supply of air. It is chiefly used in the fabrication of printer's ink. 205. What proof would you give that diamond is carbon? Jan. 1882. I should plunge the diamond into a vessel containing oxygen, after having heated it to bright redness in a cage of platinum wire; the diamond would burn with a steady red light, with the production of carbonic dioxide. I should then do the same with a piece of carbon, say charcoal, which also produces a gas which is recognised to be carbonic dioxide by the fact of its rendering lime-water milky. Thus carbon is diamond, and diamond is carbon. 206. Carbon is said to exist in three allotropic modifications. Describe why diamond, graphite, and charcoal are considered to be modifications of the element carbon. June 1864-June 1873-June 1880. If diamond (see preceding number), graphite, and charcoal are placed in hard glass tubes, and heated to redness, and oxygen passed over them, they combine with the oxygen, forming a gas which, passed over lime-water, produces the same milky precipitate of calcic carbonate (CaCO3). 207. Under what condition is diamond changed into graphite? Jan. 1882, When the diamond is introduced into the flame of the voltaic arc, it undergoes a remarkable change. At white heat the diamond begins to swell up, loses its transparency, and is converted into a black opaque mass, resembling graphite, or better coke. 208. A solid substance is suspected to contain some compound of carbon. How would you ascertain by experiment whether such is the case? Jan. 1870. Carbon may exist either as a carbonate or in some other form. If then a few drops of hydric chloride be poured over the suspected body, and a gas be given off, which, passed through lime-water, causes it to become milky or turbid, then the suspected body contains carbon and exists as a carbonate. But if no change takes place in the lime-water, mix the body with lime, and heat the mixture in a tube, and then, if the gas evolved causes the lime-water to become turbid, the suspected body contains carbon. If, however, even so no action takes place, then the suspected body contains no carbon. 209. What elements can be made to combine with carbon directly? Jan. 1882. Oxygen and sulphur at a high temperature are the only substances that combine directly with carbon. Berthollet, however, has found by experiment that by 136 igniting charcoal intensely by means of the voltaic arc in a current of pure hydrogen, a compound of hydrogen and carbon, named acetylene (C2H2), is formed. CATECHISM OF MODERN ELEMENTARY CHEMISTRY. 210. What is the action of oxygen and of sulphur separately upon red-hot charcoal? Jan. 1877. 1. If the oxygen be in excess, then carbonic dioxide is formed according to the equation:- 2. If sulphur is made to pass over red-hot charcoal, a compound is formed which is a liquid of a most unpleasant smell-namely, carbonic disulphide :— 211. How much carbon could be completely burned by a ton of air? June 1867. In a ton of air there is contained 23 % of oxygen, or 2240 × 23 100 = 515 lbs. of oxygen. Now carbon burned in oxygen produces carbonic dioxide according to the following equation: 32 lbs. of oxygen burn 12 lbs. of carbon |