24 HOW CROPS GROW. 



which it is kept, and then be wrapped tightly in several folds of paper. 

 On bringing it, thus prepared, under the mouth of the bottle, it floats 

 upward, and when the water penetrates the paper, an abundant escape 

 of gas occurs. 



Metallic iron, when at a red heat, rapidly decomposes 

 water, uniting with oxygen and setting hydrogen free, 

 as may be shown by passing steam from boiling water 

 through a gun-barrel filled with iron-turnings and heated 

 to bright redness. Certain acids which contain hydro- 

 gen are decomposed by iron, zinc, and some other metals, 

 their hydrogen being separated as gas, while the metal 

 takes the place of the hydrogen with formation of a salt. 

 Hydrochloric acid (formerly called muriatic acid) is a 

 compound of hydrogen with chlorine, and may accord- 

 ingly be termed hydrogen chloride. When this acid is 

 poured upon zinc the latter takes the chlorine, forming 

 zinc chloride, and hydrogen escapes as gas. Chemists 

 represent such changes by the use of symbols (first letters 

 of the names of chemical elements), as follows : 



H. 1^1 i rr n VT ^-'1 I ** *-vi 



HC1 + Zn_Zn cl + H or 



2 (H Cl) -\- Zn = Zn C1 2 + H, 



EXP. 12. Into a bottle fitted with cork, funnel, and delivery tubes (Fig. 

 6) an ounce of iron tacks or zinc 

 clippings is introduced, a gill 

 of water is poured upon them, 

 and lastly an ounce of hydro- 

 chloric acid is added. A brisk 

 effervescence shortly com- 

 mences, owing to the escape 

 of nearly pure hydrogen gas, 

 which may be collected in a 

 bottle filled with water as di- 

 rected for oxygen. The first 

 portions that pass over are 

 mixed with air, and should be 

 rejected, as the mixture Is dan- 

 gerously explosive. 



One of the most strik- 

 ing properties of free hy- Fig. 6. 

 flrogen is its levity. It is the lightest body in nature 



