Elements of agricultural chemistry, in a course of lectures of the Board of Agriculture / By Sir Humphry Davy.

  • Davy, Humphry, Sir, 1778-1829.
Date:
1846
    ‘160 DEFECTS OF PUBLISHED ANALYSES. The methods employed are essentially the same as those already described, and a little practice will enable the operator so to modify them as to have no unnecessary trouble. It may not be out of place here to mention, that in examining a soil with the view of ascertaining whether it contains enough of any particular body for the wants of the crop to be cultivated, that too much attention cannot be given to the state of division in which the substance exists. For analysis may shew a sufficient quantity of the body in question for a series of crops of the kind wished to be cultivated, and still, practically, the soil may be deficient in the substance. Thus, it is quite possible, that a soil containing no carbonate of lime, might be dressed with a coarse calcareous gravel to such an extent, as to shew in the analysis of the whole soil, an average quantity of calcareous matter; and yet for the requirements of the crops, be still deficient in lime. In this case, how- ever, were the impalpable portion, which is most uniformly diffused through the whole mass, and certainly presents the greatest surface for the roots to imbibe from, were this impalpable matter separated as directed in the mechanical examination, and by itself submitted to analysis, the deficiency in calcareous matter could not fail to be discovered. Indeed, the method of analysing the soil in mass is objectionable, and often leads to unsatisfactory results. Again, a constituent may be present in fine division of parts, well diffused through the whole mass of soil, and in quantity amply sufficient for the use of the crops ; and yet, unless the analysis is a very accurate, one, and made on a considerable quantity of the soil, even a good mani- pulist may fail in estimating it; and so pronounce it absent, or present in insufficient quantity. This would have been the proper place to collect together tables of analyses of soils whose history, texture, and capabilities are known, and to have deducted from them such practical conclusions as the data would afford. In the present state of our information, however, this would be unsafe; for, comparatively few foreign, and almost no British analyses of acknowledged value have yet been published. Most of the good analyses before the public, it must be regretted, are extremely deficient in the mechanical or textural part, without a knowledge of which, few practical conclusions can with safety be drawn. An additional defect in most of the published analyses, is the want of an accurate account of the ascer- tained capabilities of the soil; and perhaps a still more effectual bar to the conclusions referred to, is the want of precise information regarding the inorganic constituents of plants. It is satisfactory, however, to find that many distinguished agriculturists are now aware of the importance of accurate information on these points; and it is to be hoped, that ere long, these deficiencies will be supplied.
    LECTURE Y. ON THE NATURE AND CONSTITUTION OF THE ATMOSPHERE ; AND ITS INFLUENCE ON VEGETABLES OF THE GERMINATION OF SEEDS OF THE FUNCTIONS OF PLANTS IN THEIR DIFFERENT STAGES OF GROWTH WITH A GENERAL VIEW OF THE PROGRESS OF VEGETATION. The constitution of the atmosphere has been already generally referred to in the preceding Lectures. Water, carbonic acid gas, oxygen, and azote, have been mentioned as the principal substances composing it; but more minute enquiries respecting their nature and agencies are necessary to afford correct views of the uses of the atmosphere in vegetation. On these enquiries I now propose to enter; the pursuit of them, I hope, will offer some objects of practical use in farming; and present some philosophical illustrations of the manner in which plants are nourished; their organs unfolded, and their functions developed. If some of the salt called muriate of lime that has been just heated red be exposed to the air, even in the driest and coldest weather, it will in- crease in weight and become moist; and in a certain time will be converted into a fluid. If put into a retort and heated, it will yield pure water; will gradually recover its pristine state ; and, if heated red, its former weight: so that it is evident, that the water united to it was derived from the air. And that it existed in the air in an invisible and elastic form, is proved by the circumstance, that if a given quantity of air be exposed to the salt, its volume and weight will diminish, provided the experiment be correctly made. The quantity of water which exists in air, as vapour, varies with the temperature. In proportion as the weather is hotter, the quantity is greater. At 50° of Fahrenheit, air contains about of its volume of vapour; and as the specific gravity of vapour is to that of air nearly as 10 to 15, this is about of its weight.*' At 100°, supposing that there is a free communication with water, it contains about Vr part in volume, or in weight. It is the condensa- tion of vapour by diminution of the temperature of the atmosphere, which is probably the principal cause of the formation of clouds, and of the deposition of dew, mist, snow, or hail, f * Air at 50° F., with all the vapour it can take up, contains about g*T of its volume, which corresponds very nearly with x^5th of its weight. T It is sufficient for the precipitation of moisture, that two portions of air, of different temperatures, but each containing the full complement of vapour due to its temperature, be mixed together. The temperature speedily arrives at the x
    The power of different substances to absorb aqueous vapour from the atmosphere by cohesive attraction was discussed in the last Lecture. The leaves of living plants appear to act upon the vapour likewise in its elas- tic form, and to absorb it. Some vegetables increase in weight from this . cause, when suspended in the atmosphere and unconnected with the soil; such are the house-leek, and different species of the aloe.* In very in- tense heats, and when the soil is dry, the life of plants seems to be preserved by the absorbent power of their leaves ; and it is a beautiful circumstance in the economy of nature, that aqueous vapour is most abundant in the atmosphere when it is most needed for the purposes of life ; and that when other sources of its supply are cut off, this is most copious. The compound nature of water has been referred to. It may be proper to mention the experimental proofs of its decomposition into, and compo- sition from, oxygen and hydrogen. If the metal called potassium be exposed in a glass tube to a small quantity of water, it will act upon it with great violence; elastic fluid will be disengaged, which will be found to be hydrogen ; and the same effects will be produced upon the potassium, as if it had absorbed a small quantity of oxygen ; and the hydrogen disengaged, and the oxygen added to the potassium are in weight as 2 to 15 ; and if two in volume of hy- drogen, and one in volume of oxygen, which have the weights of 2 and 15, be introduced into a close vessel, and an electrical spark passed through them, they will inflame and condense into 17 parts of pure water, f It is evident from the statements given in the third Lecture, that wa- ter forms by far the greatest part of the sap of plants ; and that this sub- stance, or its elements, enters largely into the constitution of their organs and solid productions. Water is absolutely necessary to the economy of vegetation in its elas- tic and fluid state; and it is not devoid of use even in its solid form. Snow and ice are bad conductors of heat; and when the ground is covered with snow, or the surface of the soil or of water is frozen, the roots or bulbs of the plants beneath are protected by the congealed water from the influence of the atmosphere, the temperature of which in northern winters is usually very much below the freezing point; and this water becomes the first nourishment of the plant in early spring. The expansion of water during its congelation, at which time its volume increases iV, and its contraction of bulk during a thaw, tend to pulverise the soil; to separate mean, but the mean temperature cannot support the mean quantity of vapour, a portion of it is therefore precipitated in the form of cloud, rain, or snow, according to circumstances. Hail seems always to be produced by the falling of rain from a warmer, through a colder stratum of air. Dew is formed only on the surface of the earth; and it arises from the oooling of bodies by radia- tion ; hence, it occurs on the coldest portions of the best radiators, such as grass, &c., and only on clear nights, as then only can the temperature fall considerably. It is most abundant in tropical countries, as there the quantity of vapour in the air is greatest; and in our climate most dew occurs in autumn and spring, there being at these seasons the greatest difference between the temperature of the day and night. * Mr Macnab gives an interesting account of two fig trees (F. Australis) that were kept for many years in the Edinburgh Botanical Garden, sus- pended in air. They retained their vitality and annually put forth leaves. t One equivalent of water 9, consists of one equivalent of oxygen 8, and one of hydrogen 1.
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    gas is nearly \ heavier than the other elastic parts of the atmosphere in their mixed state : hence, at first view, it might be supposed that it would be most abundant in the lower regions of the atmosphere : but unless it has been immediately produced at the surface of the earth in some chemi- cal process, this does not seem to be the case : elastic fluids of different specific gravities have a tendency to equable mixture by a species of at- traction,* and the different parts of the atmosphere are constantly agitated and blended together by winds or other causes. De Saussure found lime water precipitated on Mount Blanc, the highest point of land in Europe ; and carbonic acid gas has been always found, apparently in due propor- tion, in the air brought down from great heights in the atmosphere by aerostatic adventurers, f The experimental proofs of the composition of carbonic acid gas are very simple. If 13 grains of well burnt charcoal be inflamed by a burning glass in 100 cubical inches of oxygen gas the charcoal will entirely dis- appear ; and provided the experiment be correctly made, all the oxygen except a few cubical inches, will be found converted into carbonic acid; and what is very remarkable, the volume of the gas is not changed. On this last circumstance it is easy to found a correct estimation of the quan- tity of pure charcoal and oxygen in carbonic acid gas : the weight of 100 cubical inches of carbonic acid gas is to that of 100 cubical inches of oxy- gen gas, as 47 to 34: so that 47 parts in weight of carbonic acid gas, Boussingault’s trials were made daily for nine months. The monthly averages vary from 1 in 2632 to 1 in 2325. The maximum, 1 in 1493, occurred on the 9th September, and the minimum, 1 in 4545, on the 10th December. Of the observed monthly means, January gave the lowest, and May and July, which gave the same result, the highest. Boussingault’s mean result above stated is of course the mean of all the daily observations. Dumas and Boussingault have recently determined with much accuracy the ratio of oxygen and nitrogen in the air. They consider their research as render- ing it probable that the air is a uniform mixture at all times in all latitudes, and at all heights, of 2301 parts of oxygen, and 7699 of nitrogen by weight; or of 20 81 of oxygen and 79'19 of nitrogen by volume. The following statement in round numbers by Dr. Clark, conveys perhaps as simple an idea of the composition of the atmosphere as can be given. 1900 Volumes of Nitrogen. 500 ... Oxygen. 1 .. Carbonic Acid. * The hypothesis of independent equilibrium originated with Dalton, and has been confirmed and extended by Professor Graham. When two vessels con- taining different gases communicate with one another, diffusion takes place till each of the gases pervades the whole space to which it has access, independently of the presence of the other. The diffusiveness of any gas is represented by the reciprocal of the square root of its density. The density being known its diffusiveness may be calculated; or if it be a gas whose diffusiveness can be accurately determined by experiment, its density may be calculated. There is a very striking analogy between the diffusion of gases through porous media, and the exosmose and endosmose of liquids through membraneous tissues men- tioned in a former note. f By moisture in the air, and by water on the surface of the earth, as well as by the influence of light on plants, this gas is removed from the air. Accord- ingly, over the sea and lakes, it is found in smaller quantity than over the land, after rain than during dry weather, in winter than in summer, and near the earth than at great elevations.