INTRODUCTION.

xviii

which wood is formed, and, indeed, all those naturaloperations in which cellular or organised matter is gene-rated under the influence of light and heat, are but veryimperfectly explained. A knowledge of the chemicalcomposition of soils, and the various substances employed j

as manures, enables us to comprehend the mode in which Jthe latter act; and a knowledge of the nature of those isubstances which plants require, points out the best andmost economical methods of restoring to the soil, by 'manures, those substances which plants remove from it ;but our knowledge of this part of the subject is veryfar from being complete, for although it is certainthat, in addition to the great elements of organic matter,which plants derive from both air and soil, they like-wise absorb small quantities of inorganic or mineralsubstances from the soil exclusively, the office per- !

formed by the latter in the vegetable economy is not >

yet well understood. Many theories indeed have been jformed respecting their use, but very little is positively iknown on the subject. I

Although Agricultural Chemistry is in this im- ‘jperfect state, and though much still remains to be j

done in that branch of science, yet it is so far ad- <

vanced as to be able to render substantial assistance tothe practical agriculturist It can teach him the prin- '