Carbonic acid is indeed produced in a great variety of ways, besides by animals in breathing, and fuel in burning. It is remarkable that it is only the green parts of plants which breathe as has been described; the leaves and stems giving out oxygen, and absorbing carbonic acid. The flowers and the ripe fruits of plants act on the air in the same way as animals, and hence deteriorate it; and the rooms where stores of fruit are kept, are known to be very unwholesome, and persons have been suffocated by sleeping in a room where there was a very great quantity of flowers. Oils, particularly drying oil, and spirit of turpentine, act on air also, absorbing oxygen and giving out carbonic acid; and the air of a newly painted house, if the doors and windows are kept close, is consequently found to be very unfit for respiration. In many countries, particularly where there are burning mountains, carbonic acid is given off from the ground, and it collects in every hollow or cave, in consequence of being much heavier than the air. There is a cave in Italy, called the Dog's Grotto, because a dog on entering it is instantly suffocated, though a man may walk in without injury. The cause is, that the cave is filled up by carbonic acid to about four feet deep; a dog, or any animal that holds its head lower than that height, breathes carbonic acid and is choked, but a man breathes the pure air which is above it, and escapes. In deep dry wells which have been neglected, carbonic acid accumulates, and workmen who go down to clean the pit are sometimes suffocated. In such cases a candle should first be let down, and if it burns, the air is fit to breathe. If the candle be extinguished, it is unsafe for an individual to descend. In the Island of Java, however, perhaps the most remarkable collection of carbonic acid is to be found. On the summit of the highest mountain there is a circular valley of considerable depth, and presenting to the eye a spectacle combining the utmost beauty and horror. The sides of the valley are clothed with the richest perennial verdure of the tropics; all the plants which grow on that fine island are there found of surpassing magnitude and beauty, but intermixed with the skeletons of tigers, wolves, and men. There is no living animal. The greatest developement of vegetable life goes hand in hand with absolute destruction to all animal existence. The natives call this place _the Valley of Death_. It is the crater of an extinct volcano. From its bottom issue perpetually watery vapour and carbonic acid, the elements which clothe its sides with vegetable riches; but the whole being an invisible lake of carbonic acid, proves instant destruction to the unwary animal that passes over its brink. Some deserters from an English regiment concealed themselves in it, and their bodies, seen through the transparent but deadly gas by which they were surrounded, verified a fact which had been previously suspected to be a fable of the natives.