90. Oxygen gas may also be obtained from black oxide of manganese, heated to redness in an iron retort, or gun barrel :

91. From red lead, heated in the same way.

92. From nitrate of potash, (common salt petre) also heated as above.

93. From the leaves of plants, exposed to the action of the sun, under a receiver filled with water, &c.

91. All combustibles burn in this gas with greatly increased splendour.

Exp. 1. Fill a glass vessel, as an open-mouthed vial, with this gas; then as it stands on the shelf of the water-bath, slide a plate of glass under its mouth and set it upright, keeping the glass in its place. Have ready a wax-taper fixed to a piece of wire; light the taper, and sliding the plate of glass hastily off, plunge it into the gas. The taper will burn with intense brilliancy.

95. If the taper be blown out, and instantly be plunged into the gas, it will be lighted again, with a slight explosion.

Obs. As tapers, ready made, cannot always be obtained, it was thought worth while to add here, that a strip of cotton cloth, or some cotton wick, dipped in melted bees-wax, and rolled moderately hard while the wax is cooling, will answer every purpose.

96. Several of the metals, when heated in a vessel of oxygen gas, inflame, and burn with great brilliancy.

2. Take some thin pieces of zinc, such as are made by melting that metal, and pouring it into water; place a small quantity of them on a spoon, or small dish, made by hammering a piece of tinned iron concave, and fixing to it a wire handle. Place in the midst of the zinc a grain, or two, of phosphorus ; then set fire to the phosphorus, by holding the dish over a candle, and instantly plunge it into a vessel of oxygen gas. The zinc will be inflamed and will burn with a beautiful white light.

3. Procure some iron, or what is better, steel-wire, or a watch-spring; wind it round a slender rod of glass, or wood, so as to coil it up in a spiral form: the turns of the wire being about the eighth of an inch apart. Then withdraw the rod, and tie

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to the end of the wire a piece of thread, dipped in melted sulphur, or bees-wax; leave the other end of the wire strait for a few inches, which must be fixed to a cork, in such a manner that the coil will hang vertically when the cork is in its place. Fill a bottle, holding about a quart, with oxygen gas, and set its mouth upwards, covering it with a plate of glass. Then light the sulphur, or wax, and introducing the coil into the bottle, put the cork in its place. Fig. 21. The iron will burn with an inconceivably brilliant light, throwing out sparks in all directions. Now and then a globule of the melted iron falls down and will break the bottle unless some sand or water be left at the bottom. See fig. 97. All bodies, by combustion in oxygen gas, acquire an addition to their weight.

Illus. If the wire, in the last experiment be nicely weighed before it is burned, and afterwards if the globules of oxide of iron be weighed, it will be found that they weigh more than the same quantity of iron did before burning.

Expl. The iron, in burning, absorbs the solid ponderous particles of the oxygen gas, which become a part of the oxide, while the light and heat, with which the gas was combined, is given out. Oxygen gas changes the colour of blood from black to red. Exp. 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 its 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 over 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.

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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 combustion, 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. 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 light 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

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OF COMMON OR ATMOSPHERIC AIR.

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 combustion.

106. The analysis of the air, then, consists in a process by which the oxygen is absorbed, while the nitrogen remains behind, and thus the diminution of bulk will show the proportions of each.

Exp. On a stand, placed in a dish of water, put a grain or two of phosphorus; set fire to the phosphorus by touching it with a hot iron, and instantly invert over it a bell glass. By the combustion of the phosphorus, all the oxygen will be absorbed, while the nitrogen remains in the receiver, and in consequence of the absorption, the water will rise in the receiver.

Obs. If the air remaining in the glass be examined by introducing into it any lighted body, as a taper, it will instantly be extinguished, showing the entire want of that principle which supports combustion. If an animal be put into it, it soon dies. 107. To determine the quantity of oxygen absorbed, an instrument is made use of, called the Eudiometer; and the art, or method of ascertaining

the purity of the atmosphere, is called Eudiometry.

pur

Obs. 1. Various instruments have been invented for this pose, and several substances have been employed to absorb the oxygen.

2. One of the most simple Eudiometers consists of a glass tube of an inch in diameter, and 24 inches long, divided into one hundred equal parts. The scale may consist of a piece of paper pasted on, after which it must be varnished to protect it from the water. The tube is closed at one end. Into such a tube, full of common air, put a piece of phosphorus, and stop the open end carefully with a cork; then heat the part where the phosphorus lies, over a lamp; it will burn rapidly. Let the tube get cold and heat it again; then when it gets cold, put the end stopped with cork under water, and withdraw it so that the water can enter and fill the space left by the loss of the oxygen. The water will rise up in the tube and show what proportion of oxygen has been absorbed, and what proportion of nitrogen remains. Experiments of this kind have determined that the atmosphere consists of 21 parts of oxygen and 79 parts of nitrogen to the 100.

3. It is remarkable, that on examining specimens of air in all countries, and from almost every part of the globe, no perceptible difference in the composition of the atmosphere has been detected.

108. The synthesis of the atmosphere consists in mixing in proper proportions the gases of which it is composed, and then submitting the mixture to the action of a burning body.

Exp. Take two bell glasses, of equal capacities, and fill the one with oxygen and nitrogen, in the proportions of 21 parts of the former, and 79 parts of the latter; fill the other with common air. Invert both at the same instant over two tapers of the same size, and burning alike. The phenomena observed in both will be exactly alike. They will both give out the same quantities of light, and will go out at the same time.

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109. The respiration of animals produces the same effect on atmospherical air, that combustion does when an animal is included in a limited quantity of air, it dies as soon as the oxygen is consumed; and no gas will maintain animal life but oxygen, or a mixture which contains it.