62 SOURCES OF CARBONIC ANHYDRIDE. a sample of river water taken above Teddington Lock, where it was in a pure condition. From these experiments it will be seen that the water from Kingston (above Teddington) is thoroughly aërated, and contains oxygen in the proper proportion to the nitrogen, which when in solution is as I to 2. At Hammersmith, the effect of the organic impurities in abstracting the oxygen begins to be evident. It is much more marked at Somerset House, whilst at Greenwich, where the condition of the river at low water is about at the worst, the oxygen has nearly disappeared. At Woolwich, it is nearly as bad, but by the time Erith is reached a great improvement is perceptible. Had the experiments been continued still lower down the river, the proportion of oxygen would have continued to increase, owing to the admixture of aërated sea-water and the absorption of oxygen due to the successive exposure of the water to the air in its onward flow. 5.-Carbonic anhydride constitutes what is termed choke-damp by miners, and it often occasions much loss of life after the occurrence of an explosion of carburetted hydrogen, or fire-damp. It frequently also accumulates in the old workings of mines, and in pits or wells. Before descending into them, it is usual to lower a lighted candle in order to ascertain whether the light will burn; if it does so, it is generally considered safe to venture down. Instances, however, are on record in which a candle was found burning in an atmosphere which, notwithstanding, contained suf ficient carbonic anhydride to cause death. When it is necessary recommended by Liebig is the simplest:-A solution of 1 part of pyrogallic acid in 6 of water is prepared, and about 40 drops of the solution is by a fresh syringe injected into the tube h, and the mixture is briskly agitated as before; the solution of potash, if oxygen be present, becomes of an intense bistre colour, and the oxygen is quickly and completely absorbed; the gas is measured a third time, and the residue is estimated as nitrogen; the difference between the second and third readings giving the volume of oxygen. A small quantity of carbonic oxide amounting to between 2 and 3 per cent. of the volume of the oxygen is always formed in this operation.-(Crace Calvert; Boussingault.) SOURCES OF CARBONIC ANHYDRIDE. 63 to enter into an atmosphere considerably charged with this gas, Graham suggests as a precaution that the mouth and nostrils be covered with a cloth containing a mixture of slaked lime and crystallized sulphate of sodium. Such a mixture is porous enough, in a layer of an inch thick, to allow the passage of sufficient air for respiration, whilst the moist lime completely absorbs the carbonic anhydride. 6.-There is also another mode in which carbonic anhydride is very largely formed, which, independently of its importance as a source of the gas, is interesting as throwing light upon its chemical nature. Whenever charcoal, or bodies which, like wood, coal, oil, or tallow, contain carbon, are burned either in oxygen or in air, carbonic anhydride is obtained abundantly; if the gas, after combustion has terminated, be agitated with lime-water, this liquid will be immediately rendered milky. Carbon may again be extracted from carbonic anhydride. If a small piece of potassium, heated till it begins to burn in air, be introduced by means of a platinum spoon into a jar of gaseous carbonic anhydride, the potassium will continue to burn with great brilliancy. Potash will be formed at the expense of the oxygen which the gas contains, whilst charcoal is liberated, as may be seen in the black particles which are suspended in the water into which the spoon is plunged after the combustion is complete. Thus carbonic anhydride is shown, both synthetically and analytically, to be a compound substance, consisting of carbon and oxygen, and its composition may be represented as follows: Carbonic anhydride is not decomposed by mere elevation of temperature, but if a succession of electric sparks be transmitted through the gas it is partially separated into carbonic oxide, and free oxygen. Sulphur, chlorine, and the halogens may be heated with the gas without decomposing it; but if heated with hydrogen, water and carbonic oxide are formed, the decomposition being represented thus, ЄO2 + H2 = CO + H2O. Carbon, and many of the metals, such as iron and zinc, also remove a portion of the oxygen from the carbonic anhydride, and convert it into carbonic oxide (357). Applications.-Sir Goldsworthy Gurney has turned the pro 64 GENERAL PROPERTIES OF THE CARBONATES. perty of extinguishing flame possessed by carbonic anhydride to an important practical account. Coal mines, at different times and from various causes, are liable to take fire, and from the vast mass of heated matter, the conflagration not unfrequently resists all the ordinary means of checking its ravages; many acres of subterranean fire are thus produced, and the workings are of necessity abandoned. Sir G. Gurney, in such cases, closes every opening into the mine but two, one for the entrance, the other for the escape of the gases, and then, by the agency of the steam jet, pours into the mine a current of impure carbonic anhydride and nitrogen, obtained by forcing a stream of air through a coke furnace into the mine, so as to fill the entire workings with the gas; he has thus on several occasions succeeded at a very small expense in extinguishing fires which have raged unsubdued for years. A very remarkable case of this kind was mentioned in the Times for May 22, 1851 :-The 'burning waste of Clackmannan,' situate about seven miles from Stirling, had been for 30 years on fire. It occurred in a seam of nine-foot coal, and extended over an area of 26 acres; yet the fire was successfully extinguished :-about 8,000,000 cubic feet of gas were required to fill the mine, and a continuous stream of impure carbonic anhydride was kept up night and day for about three weeks. The difficulty lay not so much in putting out the fire, as in cooling down the ignited mass so that it should not again burst into flame on readmitting the air. In order to effect the necessary reduction of the temperature, water was blown in along with the carbonic anhydride in the form of a fine spray or mist. Subsequently, cold air mixed with the spray was blown in, and in a month from the commencement of operations the fire was found to be completely extinguished. (350) Carbonates.-Though but a feeble acid, the radicle of carbonic acid unites with the metals and forms a numerous and important class of salts, which have till lately been regarded as monobasic; in which case (reverting to the old notation) they would contain one equivalent of the anhydride to one equivalent of the base, like carbonate of potash (KO,CO2). But in the case of the alkalies a second equivalent of the anhydride may be combined with the metallic oxide; thus with potash there is also a bicarbonate or acid carbonate (KO,HO, 2 CO). Owing to the existence of these salts, conjoined with a consideration of the properties of many of the compounds which carbonic anhydride forms with certain organic substances, the acid is now very gene GENERAL PROPERTIES OF THE CARBONATES. 4 65 rally regarded as dibasic (554), in which case its formula would be double of that formerly adopted; carbonate of calcium would then be represented as 2 CaO,С2O̟1 or ¤â¤¤; carbonate of potassium, as 2 KO,C2O, or K,Є0,; and bicarbonate or the acid carbonate of potassium, as KO,HO,C,O,, or KHЄO. The formula of all the carbonates hitherto regarded as neutral would, upon this view, be doubled, while those of the acid carbonates and some of the double carbonates would retain their former value unchanged. The carbonates, with the exception of those of the alkaline metals, are not soluble in water; but many of the insoluble carbonates, and in particular those of calcium, magnesium, barium, and strontium, may be dissolved to some extent by water charged with carbonic acid, and are deposited in a crystalline form as the gas escapes slowly from the liquid. All the carbonates are dissolved with effervescence by diluted nitric acid, and even by acetic acid: the gas which comes off is colourless, and renders lime-water turbid; it possesses the properties of carbonic anhydride, above described. The most delicate test of the presence of free carbonic acid is one of the basic salts of lead, such as the subnitrate or the subacetate, a solution of which is instantly rendered milky by the action of the acid upon it. The carbonates of the alkaline metals, when in solution, are also decomposed by acids, with effervescence; they give with salts of calcium a white precipitate, which is immediately redissolved by an acid, with effervescence. All the carbonates, with the exception of those of the alkaline metals and barium, are decomposed by prolonged ignition, the salt being decomposed into the anhydride and a metallic oxide. The carbonates have considerable tendency to combine with each other, and form double salts, like dolomite, which is a double carbonate of calcium and magnesium (MgCa2 CO). Many basic carbonates are also known; they are often hydrated compounds-such, for example, as malachite (ЄuH,,,ЄuЄ03). 2 If M and M' represent the atom of any two different metallic monads, such as potassium and sodium, the general formulæ of the carbonates will be thus indicated: : The following table represents the composition of some important carbonates : 66 CARBONATES- -CARBON. Carbonate of potassium Carbonate of sodium Acid carb. (bicarbonate) of potassium. Sesquicarb of ammonium Carbonate of barium Blue carbonate of copper 3 CaO, CO. MgO, CaO, 2 CO, CuO, CO,. CuO, HO (CuO, CO,), CuO, HỌ § II. CARBON. Є=12, or C=6. Sp. Gr. as diamond, 3'33 to 3'55; Theoretical Density of its Vapour, 0'4146; Atomic Vol. ? (351) Carbon is an elementary body of the greatest importance. It is found nearly in a state of purity in the diamond; with a larger proportion of foreign admixture it occurs in the form of graphite, and still less pure in the abundant deposits of pit coal. It is also met with in enormous quantities in combination, under a variety of forms. Independently of the quantity of it which exists diffused through the atmosphere in the state of carbonic anhydride, it is a component of the numerous varieties of carbonate of calcium and of magnesium, constituting nearly an eighth of the entire weight of the former, and more than a seventh of that of carbonate of magnesium. It is the characteristic ingredient of all substances which are termed organic; that is, of substances which are produced directly or indirectly from the vegetable or animal creation. The solid parts of plants, shrubs, and trees owe their form and solidity to this element, which is mainly supplied to them from the carbonic anhydride in the atmosphere. This action of plants upon carbonic anhydride is one of the means ordained for preserving uniformity in the composition of the air. The quantities of carbonic anhydride poured forth from the bowels of the earth, and derived from the processes of respiration and combustion, and from numerous other less apparent sources, would by degrees occasion an injurious accumulation of the gas, but for this compensating action. In solar light the leaves of plants decompose both carbonic anhydride and water, appropriating the carbon and the hydrogen of each for their own growth and nutrition, whilst a large proportion of the oxygen which these com |