222 ELEMENTS OF SOILS. 



tions in vegetable or animal life are performed without its agency. The leaves of the 

 forest trees are spread out to exhale it, and the roots fill the soil to suck up fluids which 

 contain it. The lungs of animals expand to absorb it, and vitalize the currents of blood. 

 Every organ, every tissue feels its stimulus. Every thing in nature is formed with refe- 

 rence to it. The tiny insect and the feeble worm are subjected to its action. Every living 

 being breathes it; and though of the cold-blooded class, no animal can subsist without a 

 certain quantity to which its nature is adjusted : diminisli that quantity, and the animal 

 languishes and dies ; increase it, and the animal dies from a too rapid combustion of its 

 organs. Perfectly organized bodies can not withstand the effects of oxygen, if made to 

 inhale it in a proportion greater than one to five. Inert as vegetable life seems to be, it 

 will bear no more : neither w'U it survive a dose less than nature has provided for it. 

 Rocks and soils are but oxides. One half of the solid crust of the earth is oxygen. The 

 waters and the air are combinations of it suited to the conditions of the existence of ani- 

 mated nature, and these conditions are controlled by oxygen. 



Hydrogen. This is the lightest aeriform body whose properties have been examined : it 

 is sixteen times lighter than oxygen. It is combustible, and, when slowly burned, emits a 

 pale blue flame. If oxygen and hydrogen are Ijrought together in contact with flame, 

 the combustion is instantaneous, and followed with a report loud in proportion to the 

 quantities employed. The product of the combustion is water, a result which proves 

 synthetically the composition of this fluid ; the proportions being, by volume, 2 hydrogen 

 and 1 oxygen ; or, by weight, 1 hydrogen and 8 oxygen. 



Nitrogen. This is a gas, remarkable, it is said, for its negative properties. It is lighter 

 than oxygen. Under ordinary circumstances, it is but feebly attractive of other bodies, 

 even of oxygen ; and though their temperature be raised to the highest point which we 

 can command in the furnace, they refuse to combine. If, however, the electric spark is 

 passed through a mixture of oxygen and nitrogen, combustion ensues, and nitric acid is 

 formed. Lightning is supposed to cflect a similar combination in its passage through the 

 atmosphere. Atmospheric air, which is considered a mixture of these two gases, contains 

 20 oxygen and 80 nitrogen, omitting decimals. This proportion has been regarded as 

 indicating a chemical union ; but it seems to be explained by the fact that there is no more 

 free oxygen in the universe, by which the air can be charged so as to alter the proportion ; 

 for doubtless these two gases will mix as well in any other proportion as in that which 

 composes the atmosphere. It is the proportion created, and to this organic bodies and 

 beings are fitted. 



The physical properties of the atmosphere are no less important than the chemical. Its 

 height, its density, and consequently its pressure, are subject to as little variation as its 

 composition. When in motion, its weight is diminished. It is a solvent of water, which 

 exists in its interstices as sugar in tliose of water ; and, like water, its capacity for solution 

 under given conditions is limited. If ihe atiuosphere was anhydrous, the bodies of animals 

 would be required to be anhydrous also ; but the constitution of living bodies requires a 



