

CARBON. 



49 



spiration of animals, its presence, even in a moderate 

 proportion, being soon fatal. An animal cannot live 

 in air which contains sufficient carbonic acid for 

 extinguishing a lighted candle ; and hence the prac- 

 tical rule of letting down a burning taper into old 

 wells or pits, before any one ventures to descend. 

 When an attempt is made to inspire pure carbonic 

 acid, a violent spasm of the glottis takes place, 

 which prevents the gas from entering the lungs. If 

 it be so much diluted with air, as to admit of its 

 passing the glottis, it -then acts as a narcotic poison 

 on the system. It is this gas which so often proves 

 destructive to persons sleeping in a confined room 

 with a pan of burning charcoal. Lime-water be- 

 comes turbid when brought into contact with car- 

 bonic acid, from the union of the lime with the gas, 

 and the insoluble nature of the compound thus 

 - formed. Hence, lime-water is not only a valuable 

 test of the presence of carbonic acid, but is frequently 

 used to withdraw it altogether from any gaseous 

 mixture that contains it. Carbonic acid is absorbed 

 by water. Recently-boiled water dissolves its own 

 volume of carbonic acid, at the common temperature 

 and pressure ; but it will take up much more if the 

 pressure be increased. Water and other liquids, which 

 have been charged with carbonic acid under great 

 pressure, lose the greater part of the gas when the 

 pressure is removed. The effervescence which takes 

 place on opening a bottle of ginger beer, cider, or 

 brisk champaign, is owing to the escape of carbonic 

 acid gas. Water which is fully saturated with car- 

 bonic acid gas sparkles when it is poured from one 

 vessel to another. The solution has an agreeably 

 acidulous taste, and gives to litmus paper a red 

 stain, which is lost on exposure to the air. On the 

 addition of lime-water to it, a cloudiness is produced, 

 which at first disappears, because the carbonate of 

 lime is soluble in an excess of carbonic acid ; but a 

 permanent precipitate ensues, when the free acid is 

 neutralized by an additional quantity of lime-water. 

 The water which contains carbonic acid hi solution 

 is wholly deprived of the gas by boiling. The 

 agreeable pungency of beer, porter, and ale is, in a 

 great measure, owing to the presence of carbonic 

 acid ; by the loss of which, on exposure to the air, 

 they become stale. All kinds of spring and well- 

 water contain carbonic acid, which they absorb from 

 the atmosphere, and to which they are partly in- 

 debted for their agreeable flavour. Boiled water has 

 an insipid taste, from the absence of carbonic acid. 

 Carbonic acid is always present in the atmosphere, 

 even at the summit of the highest mountains. Its 

 origin is obvious. Besides being formed abundantly 

 by the combustion of all substances which contain 

 carbon, the respiration of animals is a fruitful source 

 of it, as may be proved by breathing a few minutes 

 into lime-water. It is also generated in all the 

 spontaneous changes, to which dead animal and 

 vegetable matters are subject. The carbonic acid 

 proceeding from such sources is commonly diffused 

 equably through the air ; but, when any of these 

 processes occur in low, confined situations, as in the 

 galleries of mines or in wells, the gas is then apt to 

 accumulate there, and form an atmosphere called 

 choke damp, which proves fetal to any animals that 

 are placed in it. These accumulations take place 

 only where there is some local origin for the carbonic 

 acid ; for example, when it is generated by fermen- 

 tative processes going on at the surface of the 

 ground, or when it issues directly from the earth, as 

 happens at the grotto del Cane, in Italy, and at 

 Pyrmont, in Westphalia. 



Though carbonic acid is the product of many 

 natural operations, no increase of its quantity in the 

 atmosphere is discoverable. Such an increase ap- 



pears to be prevented by the process of vegetation. 

 Growing plants purify the air by withdrawing car- 

 bonic acid, and yielding an equal volume of pure 

 oxygen in return ; but whether a full compensation 

 for the deterioration of the air by respiration is pro- 

 duced in this way, has not, as yet, been satisfactorily 

 determined. 



Carbonic acid abounds in mineral springs, such as 

 those of Tunbridge, Carlsbad, and Saratoga. In 

 combination with Erne, it forms extensive masses of 

 rock, which occur in all countries, and in every for- 

 mation. It unites with alkaline substances, and the 

 salts so produced are called carbonates. Its acid 

 properties are feeble, so that it is unable to neutra- 

 lize completely the alkaline properties of potash, 

 soda, and lilhia. For the same reason, all the car- 

 bonates, without exception, are decomposed by the 

 muriatic and all the stronger acids; the carbonic 

 acid is' displaced, and escapes in the form of gas. 



Another gaseous compound of carbon with oxy- 

 gen, called carbonic oxide, exists, or may be obtained 

 by heating powdered chalk, or any carbonate which 

 can bear a red heat without decomposition, with iron 

 filings in a gun-barrel. It is evolved together with car- 

 bonic acid gas, from which it may be freed by agitating 

 the mixed gases with lime-water, when the carbonic 

 acid is absorbed, and the gas in question is left in a 

 state of purity. It is colourless and insipid. Lime-water 

 does not absorb it, nor is its transparency affected by 

 it. When a lighted taper is introduced into a jar of 

 carbonic oxyde, it takes fire, and burns calmly at its 

 surface, with a lambent, blue flame. It is incapable 

 of supporting respiration. A mixture of 100 mea- 

 sures of carbonic oxyde, and rather more than 50 of 

 oxygen, on being exploded in Volta's eudiometer by 

 electricity, disappear, and 100 measures of carbonic 

 acid gas occupy their place ; from which the exact 

 composition of carbonic oxyde is easily deduced. 

 For carbonic acid contains its own bulk of oxygen ; 

 and, since 100 measures of carbonic oxyde, with 50 

 of oxygen, form 100 measures of carbonic acid, it 

 follows that 100 of carbonic oxyde are composed of 

 50 of oxygen, united with precisely the same quantity 

 of carbon acid as is contained in 100 measures of 

 carbonic acid. Consequently, the composition of 

 carbonic acid being, 



By volume, 



Vapour of carbon, 100 

 Oxygen gas, . . , 100 



100 carbonic acid ens, 



By might, 

 Carbon 6 



/Oxygen, 10 



22 carbonic acid, 

 that of carbonic oxyde must be, 



By Ojlumr, 



Vapour of carbon, 100 

 Oxygen gas, ... SO 



By might, 



Carbon 



Oxygen, . . . 



100 carbonic oxyde gas, 



14 carbonic oxyde. 



Its specific gravity is 0-9721. 



The process for generating carbonic oxyde will 

 now be intelligible. The principle of the method is 

 to bring carbonic acid, at a red-heat, in contact with 

 some substance which has a strong affinity for oxy- 

 gen. This condition is fulfilled by igniting chalk, 

 or any of the carbonates, with half its weight of iron 

 filings, or of chaicoal. The carbonate is reduced to 

 its caustic state, and the carbonic acid is converted 

 into carbonic oxyde by yielding oxygen to the iron 



