374 



SCIENTIFIC RECREATIONS. 



glass case as in the illustration (fig. 373), and connect the glass with a 

 bottle of seltzer-water or a siphon. As soon as the 

 liquid enters, the carbonic acid will ascend, and this, 

 if continued for a long time, would suffocate the bird, 

 which soon begins to develop an appearance of 

 restlessness.. 



We have already remarked upon the important 

 part taken by this gas in nature, so we need only 

 mention its existence in pits and caves. There are 

 many places in which the vapour is so strong as to 

 render the localities uninhabitable. In the Middle 

 Ages the vapours were attributed to the presence of 

 evil spirits, who were supposed to extinguish miners' 

 lamps, and suffocate people who ventured into the 

 caves. In the Grotto Del Cane there is still an 



example, and certain caves of Montrouge are often filled with the gas. 



Fig. 372. Pouring out the 

 carbonic acid gas. 



Fig- 373. Experiment with carbonic c:d. 



A lighted taper held in the hand will, by its extinction, give the necessary 

 warning. Oxygen and carbon are condensed in carbonic acid, for the gas 

 contains a volume of oxygen equal to its own. If we fill a glass globe, as 

 per illustration (fig. 374), with pure oxygen, and in the globe insert two carbon 

 points, through which we pass a current of electricity, we shall find, after the 

 experiment, that if the stop-cock be opened, there is no escape of gas, and 

 yet the mercury does not rise in the tube, so the oxygen absorbed has been 

 replaced by an equal volume of carbonic acid. 



The other combination of carbon with oxygen is the carbonic oxide 

 (CO), and when a small quantity of oxygen is burnt with it it gives a blue 

 flame, as on the top of the fire in our ordinary grates. This gas is present 



