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It is remarkable, that on examining specimens of air in aU 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 mutureto 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 tiice. .
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.
Obs. Experiment shows that life cannot be sustained where a combustible body will not burn ; hence a good precaution for those who descend into wells, or other places where foul air might be suspected, would be, first to let down a burning candle. If this will not burn, certain death would attend the descent.
The atmosphere is partly supplied with oxygen by the growth of plants. See Vegetation.
OF HYDROGEN GAS.
110. The term hydrogen is derived from two Greek words, and signifies, " to become," or " produce water," because when combined with oxygen, it forms water.
111. Hydrogen, in a separate state, has never been examined ; its affinity for caloric being so strong as to give it the form of a gas, whenever it is separated from its other combinations. In the form of a gas, hydrogen is a permanently elastic fluid, possessing very peculiar properties.
By the union of its base, with the base of oxygen, water is formed, while the light and heat of both are given out. It is the lightest of all substances whose weight we are able to estimate. It is not fitted for respiration. It is obtained by the decomposition of water, from which alone it originates.
Exp. 1. Take some zinc, granulated by melting and pouring it into water, or some iron wire, or iron filings, put it into a retort or flask, fig. 23, and pour on sulphuric acid, diluted with five or six parts |of water. A violent effervescence will ensue, owing to the escape of the gas through the water. The gas may be collected in the usual manner. See figure.
Obs. The production of hydrogen by the above experiment, depends on the decomposition of the water, by means of the zinc, or iron and the sulphuric acid. The metal has an attraction for oxygen, which it obtains from the water; this forms an oxide of the metal, which is instantly dissolved by the acid ; the surface is thus left clean, and exposed to the water from which it attracts another portion of oxygen, which is dissolved as before. In consequence of the absorption of the. oxygen from the water, the hydrogen is set free, which, uniting with caloric, appears in the form of hydrogen gas.
Exp. 2. Place a gun barrel in a furnace so as to heat it red hot in the middle ; connect to one of its ends, by means of a tube, a retort placed over an Argand lamp; to the other end, fix a tube which goes under a vessel inverted in the water-bath. Make the water in the retort boil so that the steam may pass through the red hot gun barrel. Hydrogen will be produced, and will pass into the inverted vessel.
Obs. Red hot iron has a strong attraction for oxygen. In the last experiment the oxygen of the water unites to the iron, while the hydrogen being set free, unites to caloric, and is obtained in the form of hydrogen gas.
113. Hydrogen is about 13 times lighter than air. Its levity may be shown in various ways.
 

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