TSE AQntCtJLTVttAL JOtjUNAL. 



bining with water to form carbonic acid, 

 which is, however, so unstable a sub- 

 stance that it breaks up again into Avater 

 and the di-oxide on the smallest provo- 

 cation. The combustion of any organic 

 substance produces carbon di-oxide; 

 hence wood or coal fires give off large 

 volumes of this gas. It is also produced 

 during the respiration of animals by an 

 exactly similar process; the worn-out and 

 dead particles of the body are brought 

 into close contact with the air in the 

 lungs, and are burnt up by a slow pro- 

 cess of combustion, the heat produced 

 being the cause of the warmth of the liv- 

 ing body. The decay of orgAnie matter 

 in the soil also produces quantities of 

 carbon di-oxide, and the fermentation of 

 sugar is another source of the gas. Fer- 

 mentation is a process due to a low form 

 of fungus, which converts sugar, whether 

 ordinary cane sugar or that found in fruit 

 and malt, into alcohol and carbon di-oxide. 

 iSince the presence of this gas in any 

 quantity renders air unfit for breathing, 

 we can now understand the necessity for 

 free ventilation. With all these sources 

 continually in action, we might expect 

 tharthe atmosj^here would soon become 

 too full of the gas to support life, and 

 this would doubtless be the case but for 

 a kindly provision of nature whereby 

 the balance is kept up. We have seen 

 that plants consist largely of carbon, and 

 this they obtain from the air by the ac- 

 tion of their green parts, chiefly the leaves. 

 Under the influence of sunlight, these 

 have the power of absorbing the carbon 

 di-oxide, separating the carbon for their 

 own growth, and sending out the oxygen 

 again,, thus breathing in exactly the op- 

 posite manner to animals. In this way 

 the air always keeps the same composi- 

 tion. It is easy to arrange a beautiful 

 experiment to illustrate this process. 

 Take a bunch of fresh, green 

 leaves of any succulent plant, 

 Avater-cress is as good as any, put 

 them into a bottle, and All it up with fresh 

 spring water so that no air is left. The 

 bottle must then be turned upside down 

 with its mouth under water in a saucer, 

 which may be done by plunging bottle, 

 saucer, and all under the surface of water 

 in a bucket, and lifting all out together. 



Then stand the whole arrangement in the - 

 sun, and in a few hours we shall notice 

 numerous bubbles of oxygen gas clinging 

 to the leaves or perhaps collecting at the 

 top. Spring Avater always contains car- 

 bonic acid, and the leaves have been de- 

 composing it in the manner described. 

 Eain water also naturally contains car- 

 bonic acid, which though not very power- 

 ful as acids go, yet exerts considerable 

 influence in dissolving many substances 

 in the soil, and so rendering them fit for 

 the use of plants. 



Carbonates. 



Like other acids, carbonic acid forms 

 salts called carbonates, those of calcium, 

 potassium, sodium, and ammonia being 

 the most important. Being so weak an 

 acid, it is easily expelled from its salts 

 by almost any other acid. If a little 

 Avashing soda (sodium carbonate) be 

 placed at the bottom of a glass, and a 

 little acid of any kind, even vinegar, be 

 poured on it, an effervescence will imme- 

 diately commence, due to the escape of 

 the di-oxide. If the glass is covered with 

 a sheet of paper, it Avill soon be filled with 

 the gas; then put a lighted match inside, 

 and see how suddenly it is extinguished, 



' for the flame, like an animal, lives on 

 oxygen, and dies in its absence. There 

 is a very easy and instructive test for the 

 presence of carbon di-oxide, for it read- 

 ily combines with lime, forming the in- 

 soluble carbonate of calcium. If a tea- 

 spoonful of lime is put into a bottle of 

 water, well shaken up, and allowed to 

 stand till clear, we get a solution of lime 

 called lime-water. Put a little of this 

 into a glass, and breathe into it through 

 a tube of any kind for a minute; you will 



, see it turn quite thick and milky, and if 

 left to stand for a few minutes, a quan- 

 tity of white powder will settle at ,the bot- 

 tom, Avhich is chalk or calcium carbon- 

 ate. If lime water is allowed to remain 

 uncovered in the air it soon gets coated 

 with a film of the same .carbonate, owing 

 to the action of the carbon di-oxide in the 

 air. 



Questions. 



(1) Why are coals called "black dia- 

 monds?" 



