336 



THE POPULAR EDUCATOR. 



decolorising power ; but this it regains upon being calcined at a 

 low red heat. 



These properties render charcoal invaluable in the construc- 

 tion of filters ; not only does it " aerate " and decolorise the 

 water, but by oxidising any animal matter it may contain, 

 does much to render it innoxious. 



Lampblack is prepared by burning turpentine or resin with a 

 limited supply of air, and condensing the smoke ; when mixed 

 with linseed oil and soap, it forms "printers' ink." 



The oxides of carbon are : 



Carbonic oxide .... CO 

 Carbonic acid .... CO,. 



Besides these there are some vegetable acids one of the simplest, 

 oxalic acid (C a O s ), is the sour principle in sorrel. 



Carbonic Oxide (symbol, CO; atomic weight, 28; density, 

 14). This gas is best prepared either by 



1. Depriving carbonic acid of an atom of oxygen, or 



2. By adding to it an atom of carbon. 



(!) CO a = CO"; 

 (2) CO 3 + C = 2CO. 



To perform the first experiment, heat in the iron retort used to 

 prepare oxygen from manganese dioxide, a mixture of zinc or 

 iron filings and well-dried chalk, thus 



CaOCO, + Zn = CaO + ZnO + CO. 



For the other method, for zinc substitute carbon, thus 

 CaOCO, + C = CaO + 2CO. 



The heat drives off carbonic acid from the chalk, and in the 

 first case the zinc ia oxidised at 

 the expense of this CO,, and in the 

 second case the CO takes another 

 atom of carbon. 



In each instance the gas con- 

 tains a little carbonic acid, and 

 therefore to obtain it pure, it must 

 be collected over water containing 

 potash, which retains the carbonic 

 acid to form potassium carbonate. 

 Another method of preparing it is 

 to digest pulverised ferrocyanide of 

 potassium with seven or eight times 

 its weight of sulphuric acid. 



Properties. The gas is colour- 

 tasteless, inodorous, poison- 



iimnninminiiiiii ^MIIIIIIIIIIHIIIIII 



Fig. 36. 



ous. A taper introduced into it is extinguished; but the 

 gas burns where it meets the oxygen with a blue flame, forming 

 carbonic acid, thus 



CO + O = CO, 

 2 + 1 = 2. 



The numerals refer to the volumes that is, the carbonic acid 

 formed is of the same volume as the carbonic oxide burnt. This 

 may be proved by mixing in the eudiometer carbonic oxide, and 

 half its volume of oxygen ; after the spark has passed, nothing 

 but carbonic acid will remain, whose volume is that of the 

 carbonic oxide. This gas has a great affinity for oxygen, and 

 therefore is a powerful reducing agent ; in iron-smelting fur- 

 naces the reduction of the ore is chiefly due to its action. 



A solution of cuprous chloride (Cu^Cl.,), in hydrochloric 

 acid absorbs carbonic oxide, as also does melted potassium. 



Carbonic Acid (symbol, CO 3 ; atomic weight, 44 ; density, 22). 

 When limestone or chalk, which are both carbonates of lime 

 (CaOCOJ, are heated in a kiln, the heat drives off the gas (CO 3 ), 

 leaving the lime (CaO). The gas, being half as heavy again as 

 air, collects in the fire-pit, and in any hollows close by the 

 kiln; and here many a wanderer has slept his last sleep, poisoned 

 by the gas. It is also the chief product of fermentation, and 

 collects at the bottom of vats ; its presence is ascertained by 

 lowering a lighted candle, which will be extinguished if the gas 

 be present. The escape of this gas causes the effervescence of 

 wines, the froths of porter, ale, etc., and makes bread " rise." 



It is best prepared by pouring dilute hydrochloric acid on 

 chalk, or, what is better, on pieces of marble (Fig. 36). On 

 account of its density it may be collected by displacement, and 

 even may be poured from one jar into another. The reaction is 

 thus expressed : 



CaOCO a + 2HC1 = CaCl, + H 3 + CO a , 

 calcium chloride, water, and carbonic acid being the result. It 



is better not to use sulphuric acid, as the fragments of marble 

 are then coated with calcium sulphate, which is insoluble, and 

 thus the action is retarded. The gas is unable to support life, 

 and when breathed, spasm of the glottis prevents its entrance 

 into the lungs. If its presence in air exceed four per cent., it 

 acts as a narcotic poison. 



It is exhaled from the lungs, which are organs composed of 

 a membrane some 160 square 

 yards in surface, which has 

 the property of absorbing the 

 oxygen from the air inhaled, 

 and thus bringing it in con- 

 tact with the venous blood 

 beneath. This blue blood 

 owes its colour to the pre- 

 sence of carbon. The oxy- 

 gen combines with this car- 

 bon, forming carbonic acid, 

 which is exhaled. 



This may easily be proved 

 by blowing through a glass Fig. 37. 



tube into some lime water, 



or barytic water. The calcium, or barium carbonate, is formed, 

 which renders the water milky. This chemical action is not 

 only carried on in the lungs, but over the whole body through 

 the pores of the skin. If the hand be introduced into a jar 

 of oxygen standing over water, in a short time that gas will 

 be found to be converted into carbonic acid. Hence from this 

 simple experiment will be seen the imperative necessity of 

 cleanliness. 



This chemical action is the source of animal heat, and is 

 exactly that which goes on more vigorously in a coal fire. In 

 this case the oxygen entering in at the bottom of the grate 

 combines with the coal, forming carbonic acid. As this passes 

 upward through the fire, it takes another atom of carbon, 

 forming, as we have seen, carbonic oxide, which burns again 

 into carbonic acid when it reaches the top of the fire. The 

 blue, flickering flame seen over a cinder fire is carbonic oxide 

 burning into the higher oxide. 



To exhibit the presence of carbon in carbonic acid, it is only 

 necessary to pass the gas over a piece of heated potassium. 

 The arrangement is shown in Fig. 37. The metal deprives the 

 gas of its oxygen, and the liberated carbon is deposited on the 

 interior of the bulb. 



When submitted to a pressure of 35'4 atmospheres, the gas 

 becomes a liquid. This is the means by which it is accom- 

 plished. A and c (Fig. 38) are two wrought-iron vessels. A 

 mixture of water and sodium bicarbonate is introduced into A, 

 and the tube b, filled with sulphuric acid, is placed upright in the 

 " generator," A. The top is now fixed, and the tap, s, turned. 

 The vessel is inverted, being placed on stands for that purpose, 

 and the acid is emptied into the solution of sodium bicarbonate. 

 Sodium sulphate is 

 formed, and a vast 

 quantity of carbonic acid 

 gas is liberated. By 

 means of the pipe, d, 

 the generator is con- 

 nected with the " con- 

 denser," c, which is 

 packed in ice. So great 

 is the pressure of the 

 gas, that when both the 

 taps are turned the 

 liquid carbonic acid dis- 

 tils over. When the 

 operation is complete, 

 the two vessels are dis- 

 connected. A little of 



the liquid can be received on a piece of wool by turning 

 M, the pressure of the gas in c forcing the liquid up the tube. 

 So rapid is the evaporation, that the liquid on the wool is 

 frozen. When mixed with ether, a paste is formed, which 

 possesses an extremely low temperature, by which mercury is 

 at once solidified. 



Carbonic acid, though a very weak acid, forms a numerous 

 class of " carbonates." All these effervesce with dilute nitric 

 acid. 



