Durham, and Yorkshire; and in many parts of Ireland, particularly near Belfast. In general, when limestones are not magnesian, their purity will be indicated by their loss of weight in burning; the more they lose, the larger is the quantity of calcareous matter they contain. The magnesian limestones contain more carbonic acid than the common limestones; and I have found all of them lose more than half their weight by calcination.

2229. Gypsum. Besides being used in the forms of lime and carbonate of lime, calcareous matter is applied for the purposes of agriculture in other combinations. One of these bodies is gypsum or sulphate of lime. This substance consists of sulphuric acid (the same body that exists combined with water in oil of vitriol, and lime; and when dry it is composed of 55 parts of lime and 75 parts of sulphuric acid. Common gypsum or selenite, such as that found at Shotover Hill, near Oxford, contains, besides sulphuric acid and lime, a considerable quantity of water; and its composition may be thus expressed: sulphuric acid one proportion 75; lime one proportion 55; water two proportions 34.

2230. The nature of gypsum is easily demonstrated; if oil of vitriol be added to quick-lime, there is a violent heat produced; when the mixture is ignited, water is given off, and gypsum alone is the result, if the acid has been used in sufficient quantity; and gypsum mixed with quick-lime, if the quantity has been deficient. Gypsum, free from water, is sometimes found in nature, when it is called anhydrous selenite. It is distinguished from common gypsum by giving off no water when heated. When gypsum, free from water, or deprived of water by heat, is made into a paste with water, it rapidly sets by combining with that fluid. Plaster of Paris is powdered dry gypsum, and its property as a cement, and its use in making casts, depends upon its solidifying a certain quantity of water, and making with it a coherent mass. Gypsum is soluble in about 500 times its weight of cold water, and is more soluble in hot water; so that when water has been boiled in contact with gypsum, crystals of this substance are deposited as the water cools. Gypsum is easily distinguished by its properties of affording precipitates to solutions of oxalates and of barytic salts. It has been much used in America, where it was first introduced by Franklin on his return from Paris, who had been much struck with its effects there. He sowed the words, This has been sown with gypsum, on a field of lucern, near Washington; the effects astonished every passenger, and the use of the manure quickly became general, and signally efficacious. It has been advantageously used in Kent, but in most counties of England it has failed, though tried in various ways, and upon different crops.

2231. Very discordant notions have been formed as to the mode of operation of gypsum. It has been supposed by some persons to act by its power of attracting moisture from the air; but this agency must be comparatively insignificant. When combined with water, it retains that fluid too powerfully to yield it to the roots of the plant, and its adhesive attraction for moisture is inconsiderable; the small quantity in which it is used likewise is a circumstance hostile to this idea. It has been erroneously said that gypsum assists the putrefaction of animal substances, and the decomposition of manure.

2232. The ashes of saintfoin, clover, and rye-grass, afford considerable quantities of gypsum; and the substance probably is intimately combined as a necessary part of their woody fibre. If this be allowed, it is easy to explain the reason why it operates in such small quantities; for the whole of a clover crop, or saintfoin crop, on an acre, according to estimation, would afford by incineration only three or four bushels of gypsum. The reason why gypsum is not generally efficacious, is probably because most cultivated soils contain it in sufficient quantities for the use of the grasses. In the common course of cultivation, gypsum is furnished in the manure; for it is contained in stable dung, and in the dung of all cattle fed on grass; and it is not taken up in corn crops, or crops of peas and beans, and in very small quantities in turnip crops; but where lands are exclusively devoted to pasturage and hay, it will be continually consumed. Should these statements be confirmed by future enquiries, a practical inference of some value may be derived from them. It is possible that lands which have ceased to bear good crops of clover, or artificial grasses, may be restored by being manured with gypsum. This substance is found in Oxfordshire, Gloucestershire, Somersetshire, Derbyshire, Yorkshire, &c. and requires only pulverisation for its preparation.

2233. Upon the use of sulphate of iron, or green vitriol, which is a salt produced from peat in Bedfordshire, some very interesting documents have been produced by Dr. Pearson; and there is little doubt that the peat salt and the vitriolic water acted chiefly by producing gypsum. The soils on which both are efficacious are calcareous; and sulphate of iron is decomposed by the carbonate of lime in such soils. The sulphate of iron consists of sulphuric acid and oxide of iron, and is an acid and a very soluble salt; when a solution of it is mixed with carbonate of lime, the sulphuric acid quits the oxide of iron to unite to the lime, and the compounds produced are insipid and comparatively insoluble.

2234. Vitriolic impregnations in soils where there is no calcareous matter are injurious; but it is probably in consequence of their supplying an excess of ferruginous matter to the sap. Oxide of iron, in small quantities, forms a useful part of soils; it is found in the ashes of plants, and probably is hurtful only in its acid combinations. The ashes of all peats do not afford gypsum. In general, when a recent peat-ash emits a strong smell, resembling that of rotten eggs when acted upon by vinegar, it will furnish gypsum.

2235. Phosphate of lime is a combination of phosphoric acid and lime, one proportion of each. It is a compound insoluble in pure water, but soluble in water containing any acid matter. It forms the greatest part of calcined bones. It exists in most excrementitious substances, and is found both in the straw and grain of wheat, barley, oats, and rye, and likewise in beans, peas, and tares. It exists in some places in these islands native, but only in very small quantities. Phosphate of lime is generally conveyed to the land in the composition of other manure, and it is probably necessary to corn crops and other white crops.

2236. Bone-ashes calcined and ground to powder will probably be found useful on arable lands containing much vegetable matter, and may perhaps enable soft peats to produce wheat; but the powdered bone in an uncalcined state is much to be preferred in all cases when it can be procured.

2237. The saline compounds of magnesia will require very little discussion as to their uses as manures. In combination with sulphuric acid, magnesia forms a soluble salt. This substance, it is stated by some enquirers, has been found of use as a manure; but it is not found in nature in sufficient abundance, nor is it capable of being made artificially sufficiently cheap to be of useful application in the common course of husbandry.

2238. Wood-ashes consist principally of the vegetable alkali united to carbonic acid; and as this alkali is found in almost all plants, it is not difficult to conceive that it may form an essential part of their organs. The general tendency of the alkalines is to give solubility to vegetable matters; and in this way they may render carbonaceous and other substances capable of being taken up by the tubes in the radical fibres of plants. The vegetable alkali likewise has a strong attraction for water, and even in small quantities may tend to give a duc degree of moisture to the soil, or to other manures; though this operation, from the small quantities used or existing in the soil, can be only of a secondary kind.

2239. The mineral alkali or soda is found in the ashes of sea-weed, and may be procured by certain chemical agencies from common salt. Common salt consists of the metal named sodium, combined with chlorine; and pure soda consists of the same metal united to oxygen. When water is present, which can afford oxygen to the sodium, soda may be obtained in several modes from salt. The same reasoning will apply to the operation of the pure mineral alkali, or the carbonated alkali, as to that of the vegetable alkali; and when common salt acts as a manure, it is probably by entering into the composition of the plant in the same manner as gypsum, phosphate of lime, and the alkalies. Sir John Pringle has stated, that salt in small quantities assists the decomposition of animal and vegetable matter. This circumstance may render it useful in certain soils. Common salt, likewise, is offensive to insects. In small quantities it is sometimes a useful manure, and it is probable that its efficacy depends upon many combined causes. Some persons have argued against the employment of salt; because when used in large quantities, it either does no good, or renders the ground sterile; but this is a very unfair mode of reasoning. That salt in large quantities rendered land barren, was known long before any records of agricultural science existed. We read in the Scriptures, that Abimelech took the city of Shechem," and beat down the city, and sowed it with salt;" that the soil might be for ever unfruitful. Virgil reprobates a salt soil; and Pliny, though he recommends giving salt to cattle, yet affirms, that when strewed over land it renders it barren. But these are not arguments against a proper application of it. Refuse salt in Cornwall, which, however, likewise contains some of the oil and exuvia of fish, has long been known as an admirable manure. And the Cheshire farmers contend for the benefit of the peculiar produce of their county. It is not unlikely, that the same causes influence the effects of salt, as those which act in modifying the operation of gypsum. Most lands in this island, particularly those near the sea, probably contain a sufficient quantity of salt for all the purposes of vegetation; and in such cases the supply of it to the soil will not only be useless, but may be injurious. In great storms the spray of the sea has been carried more than fifty miles from the shore; so that from this source salt must be often supplied to the soil. Salt is found in almost all sandstone rocks, and it must exist in the soil derived from these rocks. It is a constituent likewise of almost every kind of animal and vegetable

manure.

2240. Other compounds. Besides these compounds of the alkaline earths and alkalies, many others have been recommended for the purposes of increasing vegetation; such

are nitre, or the nitrous acid combined with potassa. Sir Kenelm Digby states, that he made barley grow very luxuriantly by watering it with a very weak solution of nitre ; but he is too speculative a writer to awaken confidence in his results. This substance consists of one proportion of azote, six of oxygen, and one of potassium; and it is not unlikely that it may furnish azote to form albumen or gluten in those plants that contain them; but the nitrous salts are too valuable for other purposes to be used as manures. Dr. Home states, that sulphate of potassa, which was just now mentioned as found in the ashes of some peats, is a useful manure. But Naismith (Elements of Agriculture, p. 78.) questions his results; and quotes experiments hostile to his opinions, and, as he conceives, unfavorable to the efficacy of any species of saline manure. Much of the discordance of the evidence relating to the efficacy of saline substances depends upon the circumstance of their having been used in different proportions, and, in general, in quantities much too large.

2241. Solutions of saline substances were used twice a week, in the quantity of two ounces, on spots of grass and corn, sufficiently remote from each other to prevent any interference of results. The substances tried were super-carbonate, sulphate, acetate, nitrate, and muriate of potassa; sulphate of soda; sulphate, nitrate, muriate, and carbonate of ammonia. It was found, that in all cases when the quantity of the salt equalled one thirtieth part of the weight of the water, the effects were injurious; but least so in the instance of the carbonate, sulphate, and muriate of ammonia. When the quantities of the salts were one three-hundredth part of the solution, the effects were different. The plants watered with the solutions of the sulphates grew just in the same manner as similar plants watered with rain-water. Those acted on by the solution of nitre, acetate, and super-carbonate of potassa, and muriate of ammonia, grew rather better. Those treated with the solution of carbonate of ammonia grew most luxuriantly of all. This last result is what might be expected, for carbonate of ammonia consists of carbon, hydrogen, azote, and oxygen. There was, however, another result which was not anticipated; the plants watered with solution of nitrate of ammonia did not grow better than those watered with rain-water. The solution reddened litmus paper; and probably the free acid exerted a prejudicial effect, and interfered with the result.

2242. Soot doubtless owes part of its efficacy to the ammoniacal salts it contains. The liquor produced by the distillation of coal contains carbonate and acetate of amonia, and is said to be a very good manure.

2243. Soapers' waste has been recommended as a manure, and it has been supposed that its efficacy depended upon the different saline matters it contains; but their quantity is very minute indeed, and its principal ingredients are mild lime and quick-lime. In the soapers' waste, from the best manufactories, there is scarcely a trace of alkali. Lime, moistened with sea-water, affords more of this substance, and is said to have been used in some cases with more benefit than common lime.

2244. The result of Sir H. Davy's discussion as to the extent of the effects of saline substances on vegetation, is, that except the ammoniacal compounds, or the compounds containing nitric, acetic, and carbonic acid, none of them can afford by their decomposition any of the common principles of vegetation-carbon, hydrogen, and oxygen. The alkaline sulphates and the earthy muriates are so seldom found in plants, or are found in such minute quantities, that it can never be an object to apply them to the soil. The earthy and alkaline substances seem never to be formed in vegetation; and there is every reason to believe, that they are never decomposed; for, after being absorbed, they are found in their ashes. The metallic bases of them cannot exist in contact with aqueous fluids; and these metallic bases, like other metals, have not as yet been resolved into any other forms of matter by artificial processes; they combined readily with other elements; but they remain indestructible, and can be traced undiminished in quantity, through their diversified combinations.

CHAP. III.

Of the Agency of Heat, Light, Electricity, and Water, in Vegetable Culture. 2245. The particular agency of heat, light, and water in vegetation and culture has been so frequently illustrated, that it only remains to give a general idea of their natures, and to offer some remarks on electricity.

SECT. I. Of Heat and Light.

2246. The heat of the sun is the cause of growth, and its light the cause of maturity, in the vegetable kingdom. This is universally acknowledged: animals will live without or with very little light; but no plants whatever can exist for any time without the presence of this element. The agency of electricity in vegetation is less known.

2247. Two opinions are current respecting the nature of heat. By some philosophers it is conceived to be a peculiar subtile fluid, of which the particles repel each other, but have a strong attraction for the particles of other matter. By others it is considered as a motion or vibration of the particles of matter, which is supposed to differ in velocity in different cases, and thus to produce the different degrees of temperature. Whatever decision be ultimately made respecting these opinions, it is certain that there is matter moving in the space between us and the heavenly bodies capable of communicating heat; the motions of which are rectilineal: thus the solar rays produce heat in acting on the surface of the earth. The beautiful experiments of Dr. Herschel have shown that there are rays transmitted from the sun which do not illuminate, and which yet produce more heat than the visible rays; and Ritter and Dr. Wollaston have shown that there are other invisible rays distinguished by their chemical effects.

2248. Heat is radiated by the sun to the earth, and if suffered to accumulate, Dr. Wells observes, would quickly destroy the present constitution of our globe. This evil is prevented by the radiation of heat from the earth to the heavens, during the night, when it receives from them little or no heat in return. But through the wise economy of means, which is witnessed in all the operations of nature, the prevention of this evil is made the source of great positive good. For the surface of the earth, having thus become colder than the neighboring air, condenses a part of the watery vapor of the atmosphere into dew, the utility of which is too manifest to require elucidation. This fluid appears chiefly where it is most wanted, on herbage and low plants, avoiding, in great measure, rocks, bare earth, and considerable masses of water. Its production, too, tends to prevent the injury that might arise from its own cause; since the precipitation of water, upon the tender parts of plants, must lessen the cold in them, which occasions it. The prevention, either wholly or in part, of cold, from radiation, in substances on the ground, by the interposition of any solid body between them and the sky, arises in the following manner: the lower body radiates its heat upwards, as if no other intervened between it and the sky; but the loss, which it hence suffers, is more or less compensated by what is radiated to it, from the body above, the under surface of which possesses always the same, or very nearly the same temperature as the air. The manner in which clouds prevent, or occasion to be small, the appearance of a cold at night, upon the surface of the earth, is by radiating heat to the earth, in return for that which they intercept in its progress from the earth towards the heavens. For although, upon the sky becoming suddenly cloudy during a calm night, a naked thermometer, suspended in the air, commonly rises 2 or 3 degrees: little of this rise is to be attributed to the heat evolved by the condensation of watery vapor in the atmosphere, for the heat so extricated must soon be dissipated; whereas the effect of greatly lessening, or preventing altogether, the appearance of a superior cold on the earth to that of the air, will be produced by a cloudy sky, during the whole of a long night.

2249. Dense clouds, near the earth, reflect back the heat they receive from it by radiation. But similar dense clouds, if very high, though they equally intercept the communication of the earth with the sky, yet being, from their elevated situation, colder than the earth, will radiate to it less heat than they receive from it, and may, consequently, admit of bodies on its surface becoming several degrees colder than the air. Islands, and parts of continents close to the sea, being, by their situations, subject to a cloudy sky, will, from the smaller quantity of heat lost by them through radiation to the heavens, at night, in addition to the reasons commonly assigned, be less cold in winter, than countries considerably distant from any ocean.

2250. Fogs, like clouds, will arrest heat, which is radiated upwards by the earth, and if they be very dense, and of considerable perpendicular extent, may remit to it as much as they receive, Fogs do not, in any instance, furnish a real exception to the general rule, that whatever exists in the atmosphere, capable of stopping or impeding the passage of radiant heat, will prevent or lessen the appearance at night of a cold on the surface of the earth, greater than that of the neighboring air. The water deposited upon the earth, during a fog at night, may sometimes be derived from two different sources, one of which is a precipitation of moisture from a considerable part of the atmosphere, in consequence of its general cold; the other, a real formation of dew, from the condensation, by means of the superficial cold of the ground, of the moisture of that portion of the air, which comes in contact with it. In such a state of things, all bodies will become moist, but those especially which most readily attract dew in clear weather.

2251. When bodies become cold by radiation, the degree of effect observed must depend, not only on their radiating power, but in part also on the greater or less ease with which they can derive heat, by conduction, from warmer substances in contact with them. Bodies, exposed in a clear night to the sky, must radiate as much heat to it during the prevalence of wind, as they would do if the air were altogether still. But in the former case, little or no cold will be observed upon them above that of the atmosphere, as the frequent application of warm air must quickly return a heat equal, or nearly so, to that

which they had lost by radiation. A slight agitation of the air is sufficient to produce some effect of this kind; though, as has already been said, such an agitation, when the air is very pregnant with moisture, will render greater the quantity of dew, one requisite for a considerable production of this fluid being more increased by it, than another is diminished.

2252. It has been remarked, that the hurtful effects of cold occur chiefly in hollow places. If this be restricted to what happens on the serene and calm nights, two reasons from different sources are to be assigned for it. The first is, that the air being stiller in such a situation, than in any other, the cold, from radiation, in the bodies which it contains, will be less diminished by renewed applications of warmer air; the second, that from the longer continuance of the same air in contact with the ground, in depressed places than in others, less dew will be deposited, and therefore less heat extricated during its formation.

But

2253. An observation closely connected with the preceding, namely, that in clear and still nights, frosts are less severe upon the hills, than in neighboring plains, has excited more attention, chiefly from its contradicting what is commonly regarded an established fact, that the cold of the atmosphere always increases with the distance from the earth. on the contrary the fact is certain, that in very clear and still nights, the air near to the earth is colder than that which is more distant from it, to the height at least of 220 feet, this being the greatest to which experiments relate. If then a hill be supposed to rise from a plain to the height of 220 feet, having upon its summit a small flat surface covered with grass; and if the atmosphere, during a calm and serene night, be admitted to be 10° warmer there than it is near the surface of the low grounds, which is a less difference than what sometimes occurs in such circumstances, it is manifest that, should both the grass upon the hill, and that upon the plain, acquire a cold of 10° by radiation, the former will, notwithstanding, be 10° warmer than the latter. Hence also the tops of trees are sometimes found dry when the grass on the ground's surface has been found covered with dew.

2254. A very slight covering will exclude much cold. I had often, observes Dr. Wells, in the pride of half knowledge, smiled at the means frequently employed by gardeners, to protect tender plants from cold, as it appeared to me impossible, that a thin mat, or any such flimsy substance, could prevent them from attaining the temperature of the atmosphere, by which alone I thought them liable to be injured. But, when I had learned, that bodies on the surface of the earth become, during a still and serene night, colder than the atmosphere, by radiating their heat to the heavens, I perceived immediately a just reason for the practice, which I had before deemed useless. Being desirous, however, of acquiring some precise information on this subject, I fixed, perpendicularly, in the earth of a grass plot, four small sticks, and over their upper extremities, which were six inches above the grass, and formed the corners of a square, the sides of which were two feet long, drew tightly a very thin cambric handkerchief. In this disposition of things, therefore, nothing existing to prevent the free passage of air from the exposed grass, to that which was sheltered, except the four small sticks, and there was no substance to radiate heat downwards to the latter grass, except the cambric handkerchief. The temperature of the grass, which was thus shielded from the sky, was, upon many nights afterwards examined by me, and was always found higher than that of neighboring grass which was uncovered, if this was colder than the air. When the difference in temperature, between the air several feet above the ground and the unsheltered grass, did not exceed 5°, the sheltered grass was about as warm as the air. If that difference, however, exceeded 5°, the air was found to be somewhat warmer than the sheltered grass. Thus, upon one night, when fully exposed grass was 11° colder than the air, the latter was 3° warmer than the sheltered grass; and the same difference existed on another night, when the air was 14° warmer than the exposed grass. One reason for this difference, no doubt, was that the air, which passed from the exposed grass, by which it had been very much cooled, to that under the handkerchief, had deprived the latter of part of its heat; another, that the handkerchief, from being made colder than the atmosphere by the radiation of its upper surface to the heavens, would remit somewhat less heat to the grass beneath, than what it received from that substance. But still, as the sheltered grass, notwithstanding these drawbacks, was upon one night, as may be collected from the preceding relation, 8°, and upon another 11°, warmer than grass fully exposed to the sky, a sufficient reason was now obtained for the utility of a very slight shelter to plants, in averting or lessening injury from cold, on a still and serene night.

2255. The covering has most effect when placed at a little distance above the plants or objects to be sheltered, A difference in temperature, of some magnitude, was always observed on still and serene nights, between bodies sheltered from the sky by substances touching them, and similar bodies, which were sheltered by a substance a little above them. I found, for example, upon one night, that the warmth of grass, sheltered by a