Table V. Wheat grown continuously with artificial manures 565 Table VI. Experiments on the growth of barley, forty years, 1852-91 566 Table VII. Experiments on the growth of oats, 1869-78 567 Table VIII. Experiments on root crops--Swedish turnips 568, 569 Table IX. Experiments on mangel-wurzel 568, 569 Table X. Experiments with different manures on permanent meadow-land, thirty-six years, 1856-91 570 Table XI. Experiments on the growth of potatoes--average for five seasons, 1876-80 571 Table XII. Experiments on growth of potatoes (continued)-- average for twelve seasons, 1881-92 572 * * * * * INDEX 573 PART I. HISTORICAL INTRODUCTION MANURES AND THE PRINCIPLES OF MANURING. HISTORICAL INTRODUCTION. Agricultural Chemistry, like most branches of natural science, may be said to be entirely of modern growth. While it is true we have many old speculations on the subject, they can scarcely be said to possess much scientific value. The great questions which had first to be solved by the agricultural chemist were,--What is the food of plants? and,--What is the source of that food? The second of these two questions more easily admitted of answer than the first. The source of plant-food could only be the atmosphere or the soil. As the composition of the atmosphere, however, was not discovered till the close of last century, and the chemistry of the soil is a question which is still requiring much work ere we shall be in possession of anything like a full knowledge of it, it will be at once obvious that the very fundamental conditions for a solution of the question were awanting. The beginning, then, of a true scientific agricultural chemistry may be said to date from the brilliant discoveries associated with the names of Priestley, Scheele, Lavoisier, Cavendish, and Black--that is, towards the close of last century. _Early Theories on Source of Plant-food._ While this is so, and while we must regard the early attempts made towards solving this question as being, for the most part, of little scientific value, it is not without interest, from the historical point of view, to glance briefly at some of these old interesting speculations. The Aristotelian doctrine, regarding the possibility of dividing matter into the so-called four primary elements, _fire_, _air_, _earth_, and _water_, which obtained in one form or another till the birth of modern chemistry, had naturally an important influence on these early theories. _Van Helmont's Theory._