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Put a little dark-coloured blood into a vial of oxygen gas, and shake the vial, the colour will be changed to red.
Oxygen gas is the principle of acidity.
Exp. Introduce some burning sulphur under a bell glass, filled with this gas, standing in a dish, over a small quantity of water. The sulphur, as it burns, absorbs the oxygen and forms with it sulphuric acid, which is absorbed by the water; hence the water will be sour.
OF NITROGEN OR AZOTIC GAS.
98. The term Nitrogen is compounded of two Greek words which signify " to generate nitre." Azot, which is used to signify the same substance, means " life depriver," because it destroys the life of any animal which is forced to breath it.
99- Nitrogen is principally distinguished by fits negative properties. It destroys the lives of animals immersed in it, and instantly extinguishes flame. It has no sensible taste. It is a component part of all animal substances. When united to oxygen in a certain proportion it forms atmospheric air.
100. Nitrogen gas may be obtained by various methods, and from a variety of substances.
Exp. 1. Mix equal weights of iron filings and sulphur into a paste with water, and place the mixture in a proper vessel oi er water supported on a stand; then invert over it a bell glass full of common air, and allow them to stand in this situation a day or two. The paste absorbs the oxygen from the air contained under the jar, while the nitrogen remains.
2. Place on a stand in a shallow dish of water, fig. 22, a piece of phosphorus. Set fire to the phosphorus by touching it with a hot iron, and immediately invert over it a bell of glass. The phosphorus absorbs the oxygen by com
bustion, and the nitrogen remains.
3. Put some lean meat into diluted nitric acid, and by the action of the acid on the meat
nitrogen will be extricated.
101. Nitrogen gas has the following properties.
 
66 COMMON OR ATMOSPHERIC AIK.
It immediately extinguishes a lighted taper.
Exp. Fill a small jar with this gas, and set it upright with a cover on the mouth ; light a taper, and partly removing the cover, plunge it into the gas. The hght will be instantly extinguished.
102. It destroys animal life.
Exp. Confine an insect, or any animal in it and it soon dies.
OF COMMON OR ATMOSPHERIC AIR.
103. By air is meant any portion of that elastic fluid, of which the atmosphere is chiefly composed, excluding the water and other foreign substances which it contains. At this place the air is to be considered chiefly in a chemical point of view, without regard to the physical effects or properties of the atmosphere at large.
104. The air in which we live is a permanently elastic fluid ; invisible, indeed, but easily recognised by its motion, or its resistance to motion.
Obs. It is composed of two gases, oxygen and nitrogen, probably in a state of chemical combination. The oxygen composes about one-fifth, and the nitrogen four-fifths of the atmosphere. When these two gases are separated, their properties are found to be totally unlike each other; the oxygen being eminently fitted to support combustion and life, while the nitrogen extinguishes flame and destroys life.
105. As we are not acquainted with any substance which will decompose the air, by combining with the nitrogen, while the oxygen remains in its elastic state, we are obliged in all cases, to separate the two gases by such substances as abstract the oxygen from the nitrogen. All combustibles are capable of doing this, with greater or less facility. Thus during the burning of a candle, oxygen is separated from the nitrogen, and it is on this circumstance alone, that its combustion depends'. The oxidation or rusting of iron depends on the same principle, and in this respect may be considered a slow com
stion.

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