36 LESSONS IN CHEMISTRY. 



when a balloon is filled with this gas it rises up through the 

 atmosphere in the same way that a bubble of air ascends 

 to the surface of water. Formerly it was generally em- 

 ployed for filling balloons, but coal gas, which is cheaper, 

 is at present preferred for that purpose. Coal-gas, which 

 is now so extensively employed for illuminating our cities, is 

 one of those extraordinary products for which we are indebted 

 to science. It is a compound of hydrogen and carbon, and 

 in preparing it by the distillatipn of coal in large iron vessels, 

 a small quantity of nitrogen contained in the coal is also 

 driven off, which combines with a portion of the hydrogen 

 to form ammonia. In the manufacture of gas the ammonia 

 is removed by passing the impure gas through water, and 

 this "gas-water," as has already been mentioned, is employed 

 as a manure. 



34. The chemist can by various processes cause hydrogen 

 and oxygen to unite so as to form water ; thus, if when hydro- 

 gen gas is escaping from a jet like that used for burnmg coal- 

 gas, we set fire to it, and hold over the flame a common saucer, 

 soot will not be deposited as when coal-gas is burned, but the 

 saucer will be covered with drops of water. In burning, hy- 

 drogen unites with the oxygen of the atmosphere, and water is 

 produced. Water formed in an experiment in this way is 

 chemically pure, but in this pm'e state it is never met with in 

 nature. It is in fact essential for the purposes which it is designed 

 to serve that it should invariably contain other substances. One 

 of its properties is its power of dissolving gases ; some it ab- 

 sorbs in large, and others in only small proportions. Thus, 

 while the mixture of coal-gas and ammonia of the gas-works 

 is passed through it, the former escapes scarcely diminished, 

 while the latter is retained. This property of water exercises 

 an important influence upon its effects on vegetation. We 

 invariably discover in rain and snow-water the constituents 

 of the air, carbonic acid, oxygen, nitrogen, and ammonia. But 

 spring and river- water, besides containing the above gases, are 

 also contaminated with certain matters derived from the soil. 

 If we place a few drops of spring- water on a slip of glass, and 

 boil to dryness over a lamp or candle, the water is converted 

 into steam, while the solid matters which were dissolved in it 

 remain behind, in the form of a white or brown crust. It is 

 by a similar process that the chemist procures pure water. 

 He boils a quantity of spring-water to dryness in a glass or 



