Fig. 371.—Soda-water apparatus.
That even the gas introduced into seltzer-water is capable of destroying life, the following experiment will prove. Let us place a bird within a 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.
Fig. 372.—Pouring out the carbonic acid gas.
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. 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be opened, there is no escape of gas, and yet the mercury does not rise in the tube, so the oxygen absorbed has been replaced by an equal volume of carbonic acid.
Fig. 373.—Experiment with 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 in lime kilns, and is a very deadly one. We must now pass rapidly through the compounds of carbon with hydrogen, merely referring to coal for a moment as we go on.
Coal, of which we shall learn more in Mineralogy and Geology, is a combination, mechanical or otherwise, and is the result of the decomposition of vegetable matter in remote ages,—the so-called “forests,” which were more like the jungles than the woods of the present day. Moss and fern played prominent parts in this great transformation, as we can see in the Irish peat-bogs, where the first steps to the coal measures are taken.
