75
CHAPTER IY.
ORGANIC
. We have now very briefly described most ofthose substances which are of importance in studyingvegetable chemistry. Before explaining the action whichthey have on the growth of plants, it will be proper togo a little more into detail respecting the nature oforganic matter; that is to say, the various compoundsubstances which constitute the bodies of animals andplants.
221. Most of the substances hitherto described, suchas water, ammonia, carbonic acid, common salt, &c.,consist wholly of two elements, and are sometimescalled binary compounds. On the contrary, all animaland vegetable substances consist of three or four elements,and are sometimes termed ternary or quaternary com-pounds. It has been already stated that plants andanimals are composed of carbon, oxygen, hydrogen, andnitrogen (58) ; it is very important to understand clearlythe nature of the compounds formed by these elements.
222. When we endeavour to analyze a plant, that isto say, to ascertain of what it is composed, we find thatthe greater part of it consists of carbon, oxygen, hydro-gen, and a small proportion of nitrogen, combined