COMPOUNDS OF CARBON WITH <>XY<;KN AND NITROGEN 873 



cork and tube containing calcium chloride, for absorbing the water 

 formed by the oxidation of the substance. This tube is hermetically 

 connected (by a caoutchouc tube) with potash bulbs or other weighing 

 apparatus (Chap. 'V.) containing alkali destined to absorb the carbonic 

 anhydride. The increase in weight of this apparatus shows the amount 

 of carbonic anhydride formed during the combustion of the given 

 substance, and the quantity of carbon may be determined from this, 

 because three parts of carbon give eleven parts of carbonic anhydride. 



Carbonic anhydride is colourless, has a slight smell and a feeble 

 acid taste ; its density in a gaseous state is twenty- two times greater 

 than that of hydrogen, because its molecular weight is forty-four. 6 

 It is an example of those gaseous substances which have been 

 long ago transformed into all the three states. In order to obtain 

 liquid carbonic anhydride, the gas must be submitted to a pres- 

 sure of thirty-six atmospheres at 0. 7 Its absolute boiling point 

 = + 32. 8 Liquid carbonic anhydride is colourless, does not mix with 

 water, but is soluble in alcohol, ether, and oils ; at its specific gravity 

 is 0*83. When poured into a tube, which is then sealed up, liquid 

 carbonic anhydride is easily preserved, because a thick tube easily 

 resists the pressure which the liquid entails at an ordinary temperature 

 namely, about fifty atmospheres. The boiling point of this liquid lies 

 at 80 that is to say, the pressure of carbonic acid gas at that 

 temperature does not exceed that of the atmosphere. At the ordinary 

 temperature the liquid remains as such for some time under ordinary 

 pressure, on account of its requiring a considerable amount of heat for 

 its evaporation. If the evaporation takes place rapidly, especially if 

 the liquid issues in a stream, such a decrease of temperature occurs that 



6 As carbonic anhydride is one and a. half times heavier than air, it diffuses with 

 difficulty, and therefore does not easily mix with air, but sinks in it. This may be shown 

 in various ways ; for instance, it may be carefully poured from one vessel into another 

 containing air. If a lighted taper be plunged into the vessel containing carbonic 

 anhydride it is extinguished, and then, after pouring the gas into the other cylinder, it 

 will burn in the former and be extinguished in the latter. If a certain quantity of car- 

 bonic anhydride be poured into a vessel containing air, and soap-bubbles be introduced, 

 they will only sink as far as the line where the atmosphere of carbonic anhydride com- 

 mences, as this latter is heavier than the soap-bubbles filled with air. Naturally, after a 

 certain lapse of time, the carbonic anhydride will be diffused throughout the vessel, and 

 form a uniform mixture with the air, just as salt in water. 



7 This liquefaction was first observed by Faraday, who sealed up in a tube a mixture 

 of a carbonate and sulphuric acid. Afterwards this method was very considerably im- 

 proved by Thilorier and Natterer, whose apparatus is described on page 289. It is, 

 however, necessary to remark here that in working with liquid carbonic anhydride it is 

 indispensable to have good liquefying apparatus, constant cooling, and in particular a 

 rapid preparation of large masses of pure carbonic anhydride. 



3 Carbonic anhydride, having the same molecular weight as nitrous oxide, very much 

 resembles it when in a liquid state. 



