194 METALLOIDS. 



thin that the hand, passed through it, does not recog- 

 nize its presence, can be converted into a solid snow. 

 This is done by bringing into a strong iron cylinder, 

 connected by a tube with a second similar receptacle, 

 the material for making more of the gas than there is 

 room for in the two vessels. The cylinders being 

 closed, and the gas produced by the agitation of the 

 materials, it is evident -that they must burst, or the 

 gas must pack itself away in some more condensed 

 form. The second vessel is surrounded by ice, and 

 kept extremely cold, during the process. In this colder 

 vessel, the gas assumes a liquid form. Being removed 

 in this condition, one portion of the liquid evaporates 

 so rapidly as to freeze the rest. An explosive expan- 

 sion of the liquid into gas would naturally be antici- 

 pated, but this does not occur. The materials used 

 in the process are sulphuric acid and carbonate of soda. 

 478. CARBONIC OXIDE. When carbonic 



How is car- -i 



bonic oxide acid is passed through hot coals, it loses 

 half of its oxygen, and becomes carbonic 

 oxide. This takes place in coal fires. The coal in 

 the lower part of the grate, where air is plenty, 

 receives its full supply of oxygen, and becomes car- 

 bonic acid. The hot coals above, where the supply 

 of air is limited, take half of the oxygen from the 

 carbonic acid, and reduce it to this oxide, convert- 

 ing themselves partially into carbonic oxide at the 

 same time. The new gas passes on to the top of the 

 fire, and there, where air is again abundant, it burns 

 with a blue flame, and reconverts itself into carbonic 

 acid. This gas is much more poisonous than carbonic 



