CHEMISTRY. 



removing offensive vapours from air ; charcoal, 

 especially animal charcoal, also absorbs many 

 colouring- matters, and is accordingly used to 

 decolorise solutions, as in sugar-refining. Char- 

 coal, when heated in air or oxygen, burns, forming 

 carbonic acid. 



Lampblack is a form of carbon deposited from 

 the flame of gaseous or volatile compounds of 

 carbon. When compounds of carbon and hydro- 

 gen are burnt with a sufficient supply of air or 

 oxygen, the hydrogen is burnt to water, and the 

 carbon to carbonic acid ; but if there is not enough 

 oxygen for both, the carbon, or part of it, is depos- 

 ited as smoke or lampblack. Charcoal and lamp- 

 black are ' amorphous ; ' that is, are not crystalline. 

 We have, then, three distinct forms of the element 

 carbon, two crystalline diamond and graphite 

 and one amorphous. These are called ' allotropic' 

 forms of carbon ; and carbon is said to be ' dimor- 

 phous/ because it occurs in two different crys- 

 talline forms. 



Compounds of Carbon and Oxygen. Carbonic 

 acid (anhydrous carbonic acid, carbonic anhy- 

 dride) is composed of carbon and oxygen in the 

 proportion of 3 to 8 ; its formula is CO^ It is a 

 colourless transparent gas, and has a pungent 

 taste. Its specific gravity is 1-524. Water dis- 

 solves at ordinary temperatures about its own 

 volume of the gas. It can be reduced by cold 

 and pressure to the liquid state, and the liquid 

 can be frozen so as to form a solid. When the 

 gas is produced in a vessel closed at the bottom, 

 such as a brewer's vat or tan-pit (see Fermen- 

 tation, p. 332), or poured from cracks or crevices 

 into a well or mine, it remains on account of 

 its specific gravity being i^ time that of air as 

 a layer at the bottom, only slowly mixing with 

 the air by diffusion (page 306), unless it be dis- 

 turbed. Loss of life has frequently been caused 

 by persons incautiously descending into such vats 

 or pits, as carbonic acid not only does not support 

 animal life, but is directly injurious. As already 

 mentioned, atmospheric air contains, normally, 

 about 4 parts in 10,000, and this proportion may 

 be considerably increased without serious effects 

 to animal life. Some authors assert, that air 

 which contains one part of carbonic acid in 200 

 is dangerous ; but there is no doubt that when the 

 quantity reaches 2 or 3 per cent, death may be 

 produced. Carbonic acid does not support com- 

 bustion ; a lighted candle is extinguished when 

 plunged into a jar of the gas ; it must, however, be 

 remembered that a candle will burn in an atmos- 

 phere in which a man could not live, for air con- 

 taining 2 or 3 per cent, of carbonic acid will support 

 the flame of a candle, but not the life of a man. 



Carbonic acid (being an anhydrous acid) unites 

 with bases to form carbonates as normal car- 

 bonate of soda (washing-soda), Na 2 O,CO 2 or 

 Na 2 CO 3 (this salt crystallises with 10 molecules 

 of water of crystallisation) ; bicarbonate of soda 

 (baking-soda), Na40,COH/>*O, or NaHCO 3 ; 

 carbonate of lime (chalk, marble, limestone), 

 CaO,CO 2 or CaCO 3 . From carbonates we can 

 easily obtain carbonic acid by the action of any 

 strong acid, as hydrochloric acid, and as hydrated 

 carbonic acid does not exist, we obtain by this 

 action anhydrous carbonic acid and water, thus : 



CaO,CO a + 2HC1 = CaClj + H 2 O + CO 2 . 

 Other sources of carbonic acid have already been 

 mentioned. 



Carbonic oxide is composed of carbon and 

 oxygen in the proportion 3 : 4 its formula is CO. 

 It is a colourless transparent gas, of specific 

 gravity 0-968, or almost exactly the same as that 

 of nitrogen.* Carbonic oxide is very slightly 

 soluble in water, and has not been liquefied. It 

 may be prepared in various ways ; of these we 

 shall mention three, i. When carbonic acid is 

 passed through a tube containing red-hot iron 

 turnings, the iron takes half of the oxygen from 

 the carbonic acid, and carbonic oxide is produced. 

 2. When carbonic acid is passed through red-hot 

 charcoal, it takes up carbon thus CO 2 + C = 2CO. 

 This mode of formation may be seen in a common 

 fireplace, in which a clear red fire is burning ; the 

 air enters at the bottom of the grate, and its oxygen 

 combines with carbon, forming carbonic acid ; this 

 gas passing upwards through the red-hot charcoal 

 or cinders, is converted into carbonic oxide, which 

 burns with a blue flame at the top of the fire. 3. 

 When oxalic acid is heated with strong sulphuric 

 acid, it is decomposed into water, carbonic oxide, 

 and carbonic acid, thus : 



H 2 C 2 O 4 = H 2 O + CO + CO,. 



Oxalic Acid. 



Carbonic oxide is combustible, burning with a blue 

 flame, and forming carbonic acid. It is an ex- 

 ceedingly poisonous gas, much more so than 

 carbonic acid, and, along with the latter, is the 

 cause of the deaths which have frequently been 

 produced by breathing the air of a room where 

 charcoal has been burnt without a chimney or 

 proper ventilation. 



Compounds of Carbon and Hydrogen. These 

 substances are called 'hydrocarbons,' and form 

 a very large and important class of compounds. 

 When we name as specimens of the class, par- 

 affin oil, solid paraffin, benzine, oil of turpentine, 

 oil of roses, india-rubber, it will be obvious to the 

 reader that it is impossible here to describe the 

 hydrocarbons in detail ; some of the more import- 

 ant are treated of in the paper on CHEMISTRY 

 APPLIED TO THE ARTS. We shall here enu- 

 merate some of their general characters, and 

 describe two of the simplest hydrocarbons. All 

 hydrocarbons are colourless (when pure), volatile, 

 and combustible ; when sufficient oxygen is sup- 

 plied, they burn to form carbonic acid and water. 

 If the supply of oxygen is insufficient, tarry 

 matters and lamp-black are produced. The lumi- 

 nosity or brightness of the flame depends greatly 

 on the proportion of carbon, the flame being 

 generally brighter the more carbon is contained 

 in the compound. 



The two hydrocarbons which we shall here 

 describe are, marsh gas and olefiant gas. 



Marsh gas contains carbon and hydrogen in the 

 proportion 3 : 1 ; its formula is CH 4 . Of all known 

 hydrocarbons it contains the largest proportion of 

 hydrogen, and therefore gives the least luminous 

 flame. It is a colourless transparent gas, scarcely 

 soluble in water, and has not been liquefied. Its 

 specific gravity is 0-558. It is the lightest gaseous 

 hydrocarbon, and is often called ' light carburetted 

 hydrogen.' It is produced by the decomposition 



It will be observed that the molecular weight of carbonic 

 oxide is the same as that of nitrogen, 12 + 16 = 2 X 14. Their 

 specific gravity should therefore be the same (p. 318). The slight 

 discrepancy is no doubt due to these gases (as is the case with all 

 gases) deviating slightly, and not to the same extent, from Boyle'* 



laW 323 



