ground even still higher. In cold seasons, in like manner, the cooling of water goes on far more slowly than that of earth. The great body of water of the ocean thus acts as a beneficial regulator upon coast countries, as it equally moderates the heat of summer and the cold of winter, as we see in the climate of Ireland, where in favourable situations myrtles and camellias stand the winters in the open air, while the vine will not ripen thoroughly.

The warming of the air is chiefly produced through the earth or the water giving up to it a portion of the heat absorbed; like all other bodies, air becomes lighter by being heated, and hence is thrown into currents and circulation. Warm currents of air or wind freely re-deliver their heat to cooler soils and contribute essentially to their warmth, as we may readily observe in spring, when a warm thawing wind melts away ice and snow much more rapidly than continued sunshine when the air is calm and still. By ploughing and loosening a soil we render it more accessible to air, and thus it becomes heated more quickly by this, than a close, impervious soil. The winds therefore act also as equalizers or regulators of the temperature of the globe.

The influence exercised by the different temperatures of a district upon its vegetation, is shown by a mere comparative glance at the dwarf-birch and the gnarled timber of the north, and the gigantic productions of tropical countries, where, according to the attractive pictures which Humboldt sketches in his Aspects of Nature,' the monkey-bread trees (Adansonia digitata) attain a circumference of 80 to 100 feet, ferns a height of 30 or 40 feet, and palms of 160 or 180 feet, where the Aristolochia produces blossoms 4 feet in circumference, and grasses occur with joints measuring 16 feet from one knot to another. Eternal Wisdom has ordered the nature of plants in the greatest variety, so that no region of the globe where the sun's heat is capable of melting the snow, shall be devoid of these. Many require only a very slight degree of heat to awaken life, and a short summer to complete the cycle of vegetation, from germination or budding to the ripening of the seed; they form the representatives of the scanty alpine or polar flora; others ripen only under the glow of the perpendicular rays of the sun, others in the middle between these two extremes. That man is

capable of bringing about great alterations by means of gradual acclimating, is shown by the great number of vegetables at present acclimated by him; a large field is still open here, not only for the gardener, but the farmer and forester.

In estimating the productive capacity of fields, meadows or woods, the prevailing temperatures, as also the often variable distribution through the seasons, are of the greatest importance, and in many cases more than the character and composition of the soil. This however requires prolonged accurate observations of the most varied kinds, of the barometer, thermometer, the fall of rain, the individual conditions of vegetation, &c., in each particular district. Nevertheless every farmer and forester is able to furnish contributions to such a foundation, if he will establish a calendar of weather and vegetation, in which he enters the prevailing weather of his district, with the progress of growth and development of the plants he cultivates. Even the simplest observations of this kind, for example lists of the first budding of fruit-trees, the period of blossom and ripening of orchard fruits and grain, of the appearance and deposition of birds of passage, late and early frosts, &c., may serve as useful materials toward the preparation of a "climatic cultivation chart," if only conscientiously compiled and perseveringly continued.

Light.

The mysterious sympathy between the light of the sun and the vegetable kingdom is made manifest to our eyes in the bending of plants towards the light, in the sleep of leaves and flowers, and in the green colour of plants in contrast to those which have grown in the dark. The latter is intimately connected with the peculiar vegetative process of plants, which cannot go on normally without light. As already mentioned, the green parts of plants decompose carbonic acid in the presence of light, and improve the air since they give it their oxygen. This decomposition does not go on by night nor in the flowers of plants, which have not a green colour; these deteriorate the air instead of improving it. The longer plants are exposed to the influence of light the more carbonic acid they take up from the air, and the more

rapidly proceeds the vegetation. This explains the surprising rapidity of vegetation in high northern latitudes. There, where in the height of summer the sun is hardly more than a couple of hours below the horizon, grain ripens in six weeks, while in warm Italy, where the night is almost as long as the day, the maturation occupies four or five months. As the sun is indispensable, generally, for the production of healthy and vigorous plants, it likewise exercises an essential influence upon the quantity and goodness of the special constituents contained in different plants, on the proportion of sugar, oil, resin, colouring matters, &c., which they produce. Grapes, apples and other fruits grown in the shade are less sweet, mild and delicate in flavour, than such as are ripened in sun-light. In like manner we find a more aromatic taste in grapes grown on light-coloured walls reflecting the sun's rays, than in those grown on black walls which absorb the light, although the latter ripen earlier on account of the greater heating they receive. Fodder grown in the shade is not so readily eaten by animals, and contains less nourishment than that grown in the light; a similar difference exists between the grass of low-land and that of mountains, which latter, although grown in less heat, has more flavour and is more nutritious, because the sun-light is stronger and brighter on the heights than on the plains.

We also find similar differences in our forests. Wood grown in cleared spots is spongy, lighter and less enduring and less advantageous for fuel than that grown in open situations. Farmers value oak-shrubs growing on southern declivities higher than those on northern slopes, because the bark is richer in tannin and gives a more powerful tan than the latter. Pines growing on the sunny border of a forest give more resin than those in the middle of a plantation, &c.

If, as in the torrid zone, where the air is almost always clear and transparent, abundance of pure and strong light is combined with abundance of heat, and provided there is also sufficient moisture, vegetation acquires the highest vigour and luxuriance; and here only is the vitality of plants so éxalted as to produce, with the most delicious fruits, also the most delicate and most powerful balsams and spices, such as vanilla, cinnamon, cloves, &c. In our stoves we can easily produce the temperature requisite for these plants,

but not the intensity of light of those regions, and hence the products of this artificial cultivation are far inferior in size, splendour of colour and delicacy of flavour and odour, to the spontaneous children of the tropical Flora of Asia, Africa and America.

The farmer and forester must accept the light which the heavens find good to give them, much or little, bright or dull: they cannot much alter or add to it. At most, the access of light to the plants may be regulated in some degree by opening ways to it through thinning, shutting it out by cultivating shading plants, &c. The gardener, on the contrary, possesses the power, in his hot-beds and stoves, to render the light either more warming, more illuminating or more fertilizing, by using different coloured glass for his windows. Yellowish green glass transmits the luminous and chemical rays causing the growth of plants, keeping out a portion of the warming rays; this glass has been turned to the greatest account in England for Palm-houses and other stoves, where the plants very often suffer from the scorching heat of the sun. For early forcing beds, on the contrary, in which a great rise in temperature is desired, violet glass has been found the most advantageous.

ARRIVED at the conclusion of my chemical elucidations of the "Food and Feeding of Plants," I venture further to take a hasty glance backwards and another forwards, before I shake hands in farewell to the friendly reader. The Second Part of the work has been delayed longer than I formerly hoped and desired, but if I could have followed only the feeling with which I now look back, it would not even appear yet. Although I have only given what has already been often discussed, yet it was only in writing it down, with the naturally more rigid examination which one always makes of written words, that the many obscure, indefinite

and uncertain portions of our present knowledge have come distinctly before my eyes, so vividly indeed, that I could have wished to have devoted at least some years' more labour to it, in the hope thus to have been able to bring at least some parts to more satisfactory clearness and certainty. But the feeling of duty towards the many friends which the First Part found, was necessarily more powerful than this private wish. The claim of these to a completion of the beginning to which they had given a favourable reception, must of course be the only standard for me in the present case; may it also be considered as the standard in the criticism of what is offered in this concluding piece. Whether I shall be permitted hereafter to append to it a Third Part, on the "Food and Feeding of Animals," must be left for the future to decide. At present the obscure parts are still infinitely more numerous here than in the nutrition and growth of plants.

In the indicated position of the subject lies the explanation of what many perhaps have perceived with surprise, namely, that in many cases I have not entered into the practical details, for example, of the rotation of crops, tillage, classification of soils, fallows, under-dunging, &c. In these cases, in spite of individual master-works, there are not nearly enough chemical experiments and investigations in existence to furnish a trustworthy foundation for their examination in detail. To deduce general conclusions at once from isolated experiments of this kind, is scarcely anywhere more hazardous, less permissible or less trustworthy than in Agricultural Chemistry. I have therefore considered it safer and more to the purpose to discuss the general relations involved, but these as fundamentally as possible. If the intelligent practitioner examines these closely, he will doubtless easily find out the application to the individual case for himself, and probably more correctly than if theory gave him special prescriptions or propositions.

So much for retrospect, and now for the look forwards, which affords a far more enlivening prospect. In the last few years Agricultural Chemistry has had to rejoice in a most extraordinary growth, namely, a growth in friends and amateurs; but it has not been able to advance in equal degree in its own individual growth, since its means did not