Page:The American Cyclopædia (1879) Volume IV.djvu/280

 272 CHARCOAL sure of the gas ; the lower the former and the greater the latter, the greater the absorption. In function of pressure alone, the temperature being constant, the same charcoal absorbs volumes of gas nearly, but a little less than, in the direct ratio of the pressure. The amount of absorption differs greatly for the charcoal of different woods, other things being equal ; and for the temperature at which it was formed from the same wood, the lower that temperature the greater the absorption. If charcoal saturated with one gas be exposed to another gas, it will discharge a portion of the former and absorb a portion of the latter, so that it will contain both gases, but not in so great quantities of each as if it were exposed to each alone. Recently prepared charcoal exposed to the atmosphere absorbs the oxy- gen of the air to a greater extent than the nitrogen. Damp charcoal does not absorb gases so readily nor to the same quantity as that which is dry. Charcoal loses its property of absorption by use, but regains it completely by simple reheating. The absorption of gases is always accompanied by a rise of tempera- ture in the mass. If dry charcoal saturated with sulphuretted hydrogen be introduced into oxygen, the gases combine with explosion and the production of aqueous vapor and sul- phurous acid gas ; just previous to the explo- sion a yellow vapor, that of the separated sulphur, exudes from the pores of the charcoal. If nitrogen be mixed with the oxygen, the above effect is produced more slowly, and the hydrogen alone of the sulphuretted hydrogen is combined, its sulphur being precipitated as a solid. The absorption by charcoal of gases and vapors attains its maximum in about 36 hours ; and damp charcoal does not absorb them so readily as that which is dry. This absorbent property of charcoal is ascribed to catalysis, and resembles the action of spongy platinum on certain mixtures of gases; but there is this difference between the two sub- stances, that platinum has the greater com- bining power, while charcoal has the greater absorbing power. Coarsely powdered char- coal boiled in a solution of the chloride of platinum until thoroughly soaked, and then heated to redness in a closed crucible, retains a portion of the platinum, and possesses remark- able powers of absorption and combination. Charcoal has a great deoxidizing power even at low temperatures ; it is, in fact, the great re- ducing agent of metallurgists. Fresh-burned charcoal of boxwood, free of ashes, introduced into nearly neutral and very dilute solutions of gold, platinum, palladium, silver, and copper, precipitates the metals, which are deposited on the charcoal in thin films. The deposit of copper, if allowed to remain in the liquid, soon disappears. Zinc, iron, lead, and mercury are precipitated in the same manner, but redissolve in acid liquors. At a red heat charcoal de- oxidizes many fixed and volatile metallic ox- ides (arsenious acid), reproducing their metal- lic bases. At a white heat it deoxidizes even potassa, soda, and phosphoric acid, setting free in vapor potassium, sodium, and phos- phorus. Charcoal produces in the cold a vio- lent explosion with perchloric acid ; and at a red heat it deoxidizes the chlorates, perchlo- rates, and nitrates, with deflagration, produ- cing carbonates with the respective alkaline bases. At a full red heat it converts most of the sulphates to sulphites, and it decomposes water; but it has no action on the haloid salts, namely, the chlorides, bromides, iodides, and fluorides ; neither does it decompose, even at a very high temperature, silica and alumina. When the vapor of water is passed slowly over charcoal heated to full redness in a porcelain tube, the resulting gas, called water gas, is composed by weight, in 100 parts, of hydrogen 56'03, carbonic oxide 29*15, carbonic acid 14'65, and carburetted hydrogen 0'17. The absorbing and deoxidizing powers of charcoal are greatly diminished by saturation with water. Charcoal has also a strong deodorizing power, referable to the same property. It' air containing sulphuretted hydrogen be agitated with powdered charcoal, or if water contain- ing sulphuretted hydrogen be filtered through charcoal, the offensive odor of that gas is speedily removed. The sulphuretted hydro- gen is first absorbed, and then, by a catalytic action, its hydrogen unites with the oxygen of the atmosphere, while its sulphur is de- posited. Charcoal absorbs coloring matters. Powdered charcoal agitated with solutions of dilute sulphate of indigo, of cochineal, of the blue iodide of starch, and of the red perman- ganate of potassa, entirely removes these col- ors. Port wine, by a similar process, is ren- dered tawny or light-colored ; and impure solutions of sugar and nitre lose their color by filtration through a mass of charcoal. The taste of liquids in some cases is almost removed by filtration through charcoal when the taste depends on the presence of certain organic substances. The bitterness of the hop in ale is thus removed, and strychnia, too, is com- pletely absorbed. The antiseptic power of charcoal is due to the action of its absorbed oxygen upon organic matter, under the influ- ence of which the decomposition of such mat- ter is quickened instead of retarded. Char- coal, by its possession of these properties of absorption, decomposition, and combination, is eminently fitted as a filter for the purification of water, removing from that liquid the color, odor, and taste of its impurities, by oxidizing and recombining them into other and inoffen- sive substances. The following gases stand in their respective order as regards their volumes at standard temperature and pressure, which are absorbed at saturation by dry charcoal ; the first named being absorbed in least vol- umes, and the volumes increasing for each gas to the last named; the absolute number of volumes in each case varies enormously with the charcoal of different woods and with the