[297] SCIENTIFIC MANUAL. 11 



mix, mechanically. If, however, a spark of electricity or 

 a lighted taper be brought in contact with them, there will 

 be a violent explosion, as the result of the active chemical 

 union of the two gases forming, first, watery vapor, which, 

 being immediately condensed, causes a vacuum, into which 

 air rushes with great force, producing the report. 



Electricity will also decompose water, re-converting it 

 into the two gases, which, if separated, will be found to be 

 in the proportion of two of hydrogen to one of oxygen. 

 The specific gravity of hydrogen, compared with that of 

 common air, is as 69 to 1,000. It is sixteen times lighter 

 than oxygen. In combination with nitrogen, it forms 

 ammonia. 



Water, nature's great solvent, is the most abundant con- 

 stituent in all vegetation, and is indispensable, both to the 

 germination and growth of plants, and to the existence of 

 animal life. It is the necessary vehicle of all that portion 

 of plant-food which is derived from the soil. 



It is composed of oxygen and hydrogen, chemically 

 combined in the proportion of two atomic weights of 

 hydrogen, to one of oxygen. It exists, principally, as a 

 liquid, but takes also a gaseous and solid form. It is 

 liquid at ordinary temperature, boils at 212 F., and is 

 rapidly converted into steam or vapor, evaporation taking 

 place more gradually at lower temperatures, dependent 

 upon motion and dryness of the surrounding medium. 



It freezes at 32 F., becoming solid in the form of ice. 

 Its greatest density is at 40 F., diminishing in density be- 

 low that temperature until it becomes solid, and dimin- 

 ishing above with the increase of heat. The expansion in 

 freezing or solidifying, which is a*n exception to a general 

 law of nature, is a wise provision, which not only prevents 

 the disastrous consequences which would result from an 

 increase of density in solidification in cold climates, but 

 serves a valuable purpose in agriculture : by its expansion 

 in freezing, and the resulting pulverization of rocks and soils. 



