THE AIR.

95

plants can derive from the air alone the greater part ofthe substances which they require.

290. Although the air contains so large a proportionof oxygen, and although that substance is in a free state,—that is to say, not combined with any element, butready to combine with any substance for which it hasan affinity,—yet it does not seem that plants derive theoxygen which they contain directly from the air; and,in the same way, there is no evidence to show that theyare able to absorb nitrogen from the air : it appears thatplants can only absorb these substances when in a stateof combination.

291. It might have been supposed that plants wouldobtain the nitrogen which they require directly fromthe air, which contains nearly four-fifths of that gas ;but there is very good reason to believe that this is notthe case, and that plants can only obtain nitrogen, orassimilate it, as chemists say, by absorbing it in com-bination with some other substance.

292. What has just been said with regard to oxygenand nitrogen, is equally applicable to carbon and hydro-gen : the former is a solid substance, and therefore, asone might rightly conclude, plants cannot absorb it inthe separate state ; when combined with oxygen in theform of carbonic acid gas, and probably also when inthe form of carburetted hydrogen (80), it can be ab-sorbed by plants. Hydrogen has never been found inthe air, except in a state of combination. It may belaid down as a rule, that plants can only absorb oxygen,