What is carbonic acid gas? - A gas formed by the union of carbon and oxygen. It used to be called "fixed air.

2939. Under what circumstances does carbon most readily unite with oxygen ? - 1. When its temperature is raised: thus, if carbon be red-bot, oxygen will most readily unite with it; and 2. When it forms part of the fluid blood.

2940. Why do oxygen and carbon so readily unite in the blood? - Because the atoms of carbon are so loosely attracted by the other materials of the blood, that they unite very readily with the oxygen of the air inhaled.

2941. 16 carbonic acid wholesome?- No: it is fatal to animal life: and (whenever it is inhaled) acts like a narcotic poison - producing drowsiness, which sometimes ends in death.

2942. How can any one know if a place be infested with carbonic acid gas ? - If a pit or well contain carbonic acid, a candle (let down into it) will be instantly extinguished. The rule, therefore, is this - Where a candle will burn, a man can live; but what will extinguish a candle, will also destroy life.

2943. Why does a miner lower a candle into a mine before he descends ? - Because the candle will be extinguished, if the mine contains carbonic acid gas; but if the candle is not extinguished, the mine is safe, and the man may fearlessly descend.

2944. Why does a crowded room produce head-ache? - Because we breathe air vitiated by the crowd.

2945. Why is the air of a room vitiated by a crowd? - Because it is deprived of its due proportion of oxygen, and laden with carbonic acid.

2946. How is the air of a room affected thus by a crowd? - The elements of the air (inhaled by the breath) are separated in the lungs; - the oxygen is converted in the blood into carbonic acid; and the carbonic acid (together with the nitrogen) is then thrown off by the breath into the room.

2947. Is all the nitrogen rejected by the lungs? - Yes; all the nitroge- of the air is always expired.

2948. Why is a crowded room unwholesome! - Because the oxygen of the air is absorbed by the lungs: and carbonic acid gas (which is a noxious poison) is substituted for it.

2949. Mention the historical circumstances, so well known in connexion with the "Black Hole of Calcutta."- In the reign of George II. the Raja (or Prince) of Bengal marched suddenly to Calcutta, to drive the English from the country: as the attack was unexpected, the English were obliged to submit, and 146 persons were taken prisoners.

2950. What became of these prison-ers? - They were driven into a place about eighteen feet square, and fifteen or sixteen feet in height, with only two small grated windows. 123 of the prisoners died in one night; and (of the twenty-three who survived) the larger portion died of putrid fevers after they were liberated.

2951. Why were 123 persons suffocated in a few hours, from confinement in this close, hot prison-hole? - Because the oxygen of the air was soon consumed by so many lungs, and its place supplied by carbonic acid, exhaled by the hot breath.

2952. Why did the captives in the Black Hole die sleeping? - 1. Because the absence of oxygen quickly affects the vital functions, depresses the nervous energies, and produces a lassitude which ends in death: and 2. The carbonic acid gas (being a narcotic poison) produces drowsiness and death in those who inhale it.

2953. Why are the jungles of Java and Hindostan so fatal to life ? - Be-cause vast quantities of carbonic acid are thrown off by decaying vegetables; and (as the wind cannot penetrate the thick brushwood to blow it away) it settles there, and destroys animal life.

2954. Why do persons in a crowded church feel drowsy? - 1. Because the crowded congregation inhale a large portion of the oxygen of the air which alone can sustain vitality and healthy action: and 2. The air of the church is impregnated with carbonic acid gas, which (being a strong narcotic) pro duces drowsiness in those who inhale it.

2955. Why do persons who are much in the open air, enjoy the best health? - Because the air they inhale is much more pure.

2956. Why is country air more pure than the air in cities? - 1. Because there are fewer inhabitants to vitiate the air: 2. There are more trees to restore the equilibrium of the vitiated air: and 3. The free circulation of air keeps it pure and wholesome; (in the same way as running streams are pure and wholesome, while stagnant waters are the contrary.)

2957. Why does the scantiness of a country population render the country air more pure? - Because the fewer the inhabitants the less carbonic acid will be exhaled; and thus country people inhale pure oxygen, instead of air impregnated with the narcotic poison, called carbonic acid gas.

2958. Why do trees and flowers help to make country air wholesome ? -1. Because trees and flowers absorb the carbonic acid generated by the lungs of animals, putrid substances, and other nexious exhalations: and 2. Trees and flowers restore to the air the oxygen, which has been inhaled by man and other animals.

2959. Why is the air of cities less wholesome than country air? - 1. Because there are more inhabitants to vitiate the air: 2. The sewers, drains, bins, and filth of a city, very greatly vitiate the air: 3. The streets and alleys prevent a free circulation: and 4. Besdies all this, there are fewer trees to absorb the excess of carbonic acid gas, and restore the equilibrium.

2960. Why are persons who live in close rooms and crowded cities generally sickly? - Because the air they breathe is not pure, but is (in the first place) defective in oxygen: and (in the second) impregnated with carbonic acid gas.

2961. Where does the carbonic acid of close rooms and cities come from? -From the lungs of the inhabitants, the sewers, drains, and other like places, in which organic substances arc undergoing decomposition.

2962. What becomes of the carbonic acid of c\owded cities? - Some of it is absorbed by vegetables; and the rest is blown away by the wind, and diffused through the whole volume of the air.

2963. Does not this constant diffusion of carbonic acid affect the purity of the whole air? - No; because it is wafted by the wind from place to place, and absorbed in its passage by the vegetable world.

2964. What is choke damp? - Carbonic acid gas accumulated at the bottom of wells and pits, which renders them noxious, and often fatal to life.