of the carbonic acid, either by dissolving it, or causing the soil to do so. It was found, that during the night, the quantity of carbonic acid was greater than that of the day, in the proportion of 4.32 to 3.98; but if the wind were strong,then scarcely any difference occurred. The greater quantity of carbonic acid occurring in the night, is attributed to the want of decomposition, which arises from vegetation during the day; a result which favours the explanation of the above question. A short frost, which does not penetrate the earth to more than ⚫ an inch, does not appear to cause any variation in the quantity of carbonic acid; but when the frost continues long, the dryness which it occasions, increases the proportion. It was also found, that the quantity of carbonic acid is greater during the day in a city than in the country, that the variations occasioned by the seasons are analogous, and that the quantity of carbonic acid increases more by the influence of night in the country than in the town. The air of the plains contains more carbonic acid than that of the mountains; which difference is explained by considering that the decomposition of the acid occurs principally where vegetation is most abundant, as it is in the plains, and that the gas is absorbed by the earth there, because it contains more rain-water.—Abridged from La Bibliotheque Universelle.

Why is a blue flame so often seen upon the surface of a charcoal fire?

Because the combustion of the carbonic oxide is formed in this way: the air entering at bottom, forms carbonic acid, which, passing through the red hot charcoal, becomes converted into carbonic oxide. Hence arises the danger of burning charcoal in ill ventilated chambers.

Why is carbonic acid frequently found at the bottom of mines, wells, &c.?

Because it is much heavier than atmospheric air.

Workmen ought never to venture into such places without previously letting down a lighted candle. If the candle burns, they may enter safely; if not, quicklime should be let down in buckets, and gradually sprinkled with water. As the lime slacks, it will absorb the carbonic acid gas, and the workmen may af terwards descend in safety.-Parkes.

In these cases, the carbonic acid issues from crevices in the earth, and is produced by unknown sources. Similar accidents happen to persons incautiously descending into brewers' vats, before they have been purified from this gas. We were told of three or four such cases at the brewhouse of Messrs. Barclay and Perkins, as we were walking over their stupendous store-vats a few years since.

The noxious properties of carbonic acid, or chokedamp, in mines, have already been noticed in connexion with the Safety Lamp. (See p. 54, et seq.)

Why do persons experience difficulty of breathing, giddiness, and faintness, in crowded and illuminated rooms, which are ill-ventilated?

Because of the excess of carbonic acid which is always produced by the respiration of animals. In like manner, some manufactories are very unhealthy; as all common combustibles, such as coal, wood, oil, wax, tallow, &c. contain carbon as one of their component parts; so the combustion of these bodies is always attended by the production of carbonic acid. It is not, therefore, surprising, that carbonic acid should be always present in the atmosphere. Indeed, the great wonder is, considering the abundance of its sources, that it does not accumulate to an injurious degree, as it certainly would, without one of those admirable and providential compensations which perpetually strike us among the laws of nature; by which it is provided, that the process of vegetation should remove the contamination produced by the animal part.

Carbonic oxide, when respired, is fatal to animal life. Sir Humphry Davy took three inspirations of it mixed with about one-fourth of common air; the effect was a temporary loss of sensation, which was succeeded by giddiness, sickness, acute pains in different parts of the body, and extreme debility. Some days elapsed before he entirely recovered. Since then, Mr. Witter of Dublin was struck down in an apoplectic condition by breathing this gas; but he was speedily restored by the inhalation of oxygen.

Why have workmen often lost their lives by sleeping too near limekilns ?

Because carbonic acid gas is extricated from the kilns in great abundance.

This brings to our recollection an incident, which occurred a few years back at a limekiln adjoining the old Bristol road, and manifests how perfectly insensible the human frame may be to pain in peculiar circumstances. It is related by Mr. Leonard Knapp, in his interesting Journal of a Naturalist, and though not exclusively illustrative of the above question, we are induced to quote his narrative:-" A travelling man one winter's evening laid himself down upon the platform of a limekiln, placing his feet, probably numbed with cold, on the heap of stones newly put on to burn through the night. Sleep overcame him in this situation; the fire gradually rising and increasing, until it ignited the stones upon which his feet were placed. Lulled by the warmth, he still slept; and though the fire increased until it burned one foot, (which probably was extended over a vent-hole) and part of the leg, above the ancle, entirely off, consuming that part so effectually, that no fragment of it was ever discovered -the wretched man slept on! and in this state was found by the kiln-man in the morning. Insensible to any pain, and ignorant of his misfortune, he attempted to rise and pursue his journey, but missing his shoe,