COMPOUNDS OF CARBON WITH OXYGEN AND NITROGEN 385 



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 

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

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

 ^ 0-83. 8bis 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 

 a part of the carbonic anhydride is transformed into a solid snowy 

 mass. Water, mercury, and many other liquids freeze on coming into 

 contact with snow-like carbonic anhydride. 9 In this form carbonic 

 anhydride may be preserved for a long time in the open air, because it 

 requires still more heat to turn it into a gas than when in a liquid 

 state. 9 bis 



The capacity which carbonic anhydride has of being liquefied stands 



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 given in Chapter VI. in describ- 

 ing NjO. It is, however, necessary to remark that the preparation of liquid carbonic 

 anhydride requires good liquefying apparatus, constant cooling, and a rapid preparation 

 of large masses of carbonic anhydride. 



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

 resembles it when in a liquid state. 



8 bis "When poured into a tube, which is then sealed up, liquefied carbonic anhydride 

 can be easily preserved, because a thick tube easily supports the pressure (about 50 

 atmospheres) exerted by the liquid at the ordinary temperature. 



9 When a fine stream of liquid carbonic anhydride is discharged into a closed metallic 

 vessel, about one-third of its mass solidifies and the remainder evaporates. In employing 

 solid carbonic anhydride for making experiments at low temperatures, it is best to use it 

 mixed with ether, otherwise there will be few points of contact. If a stream of air be 

 blown through a mixture of liquid carbonic anhydride and ether, the evaporation proceeds 

 rapidly, and great cold is obtained. At present in some special manufactories (and for 

 making artificial mineral waters) carbonic anhydride is liquefied on the large scale, filled 

 into wrought-iron cylinders provided with a valve, and in this manner it can be trans- 

 ported and preserved safely for a long time. It is used, for instance, in breweries. 



9 bls Solid carbonic anhydride, notwithstanding its very low temperature, can be 

 afely placed on the hand, because it continually evolves gas which prevents its coming 

 into actual contact with the skin, but if a piece be squeezed between the fingers, it pro- 

 duces a severe frost bite similar to a burn. If the snowlike solid be mixed with ether, a 

 semi-liquid mass is obtained, which is employed for artificial refrigeration. This mixture 

 may be used for liquefying many other gases such as chlorine, nitrous oxide, hydrogen 

 sulphide, and others. The evaporation of such a mixture proceeds with far greater 



