XVI

INTRODUCTION.

It lias long been known, that plants consist of Carbon .Oxygen, Hydrogen, and Nitrogen , and also that theyinvariably contain a small quantity of inorganic, orearthy and saline, matters. The presence of Nitrogen was formerly greatly overlooked, in analyses of vegetablesubstances ; it is contained in less quantity than theother three elements of organic matter, and was verycommonly regarded as being merely accidental, and nota necessary constituent of plants. Improved modes ofanalysis have established that it always exists in thesame proportion, in certain constituents of plants ; andas it appears that these substances are also those whichform the most valuable part of food, it becomes a ques-tion of the first importance, whence do the plants derivetheir Nitrogen ? They obtain it principally, if not wholly,from the air ; they do not absorb it in the free and un-combined form, but they absorb it combined withHydrogen, in the state of Ammonia. The importance ofthe earthy substances in plants was, likewise, greatlyoverlooked formerly. It has been proved, by repeatedexperiments, that these substances are of the greatestimportance in the growth of plants, being quite essen-tial to their developeinent.

Although much has been done, and although chemistshave laboured to remove the perplexities which encom-