mometer, and their heating power was observed. The heating power of the violet, green, and red rays were found to be to each other as the following numbers: :- - Violet, 160; Green 22.4; Red, 550. The heating power of the most refrangible rays was least, and this power increases as the refrangibility diminishes. The red ray, therefore, has the greatest heating power, and the violet, which is the most refrangible, the least. The illuminating power, it has been already observed, is greatest in the middle of the spectrum, and it diminishes towards both extremities; but the heating power, which is least at the violet end, increases from that to the red extremity; and when the thermometer was placed beyond the limit of the red ray, it rose still higher than in the red ray, which has the greatest heating power in the spectrum. The heating power of these invisible rays was greatest at the distance of half an inch beyond the red ray, but it was sensible at the distance of one inch and a half.

2326. The influence of the different solar rays on vegetation has not yet been studied; but it is certain that the rays exercise an influence independent of the heat they produce. Thus plants kept in darkness, but supplied with heat, air, and moisture, grow for a short time, but they never gain their natural colours; their leaves are white and pale, and their juices watery and peculiarly saccharine: according to Knight they merely expend the sap previously generated under the influence of light. (Notes to Sir H. Davy's Agr. Chem. p. 402.)

2327.

SECT. II. Of Electricity.

Electrical changes are constantly taking place in nature, on the surface of the earth, and in the atmosphere; but as yet the effects of this power on vegetation have not been correctly estimated. It has been shown by experiments made by means of the voltaic battery, that compound bodies in general are capable of being decomposed by electrical powers; and it is probable that the various electrical phenomena occurring in our system, must influence both the germination of seeds and the growth of plants. It has been found that corn sprouted much more rapidly in water positively electrified by the voltaic instrument, than in water negatively electrified; and experiments made upon the atmosphere show that clouds are usually negative; and, as when a cloud is in one state of electricity, the surface of the earth beneath is brought into the opposite state, it is probable that in common cases the surface of the earth is positive. A similar experiment is related by Dr. Darwin. (Phytologia, sect. xiii. 2, 3.)

2328. Respecting the nature of electricity different opinions are entertained amongst scientific men. By some, the phenomena are conceived to depend upon a single subtile fluid in excess in the bodies said to be positively electrified, and in deficiency in the bodies said to be negatively electrified; a second class suppose the effects to be produced by two different fluids, called by them the vitreous fluid and the resinous fluid; and others regard them as affections or motions of matter, or an exhibition of attractive powers similar to those which produce chemical combination and decomposition, but usually exerting their action on masses.

2329. A profitable application of electricity, Dr. Darwin observes, to promote the growth of plants is not yet discovered; it is nevertheless probable, that, in dry seasons, the erection of numerous metallic points on the surface of the ground, but a few feet high, might in the night time contribute to precipitate the dew by facilitating the passage of electricity from the air into the earth; and that an erection of such points higher in the air by means of wires wrapped round tall rods, like angling rods, or elevated on buildings, might frequently precipitate showers from the higher parts of the atmosphere. Such points erected in gardens might promote a quicker vegetation of the plants in their vicinity, by supplying them more abundantly with the electric ether. (Phytologia, xiii. 4.) J. Williams (Climate of Great Britain, 348.), enlarging on this idea, proposes to erect large electrical machines, to be driven by wind, over the general face of the country, for the purpose of improving the climate, and especially for lessening that superabundant moisture which he contends is yearly increasing from the increased evaporating surface, produced by the vegetation of improved culture, and especially from the increase of pastures, hedges, and ornamental plantations.

SECT. III. Of Water.

2330. Water is a compound of oxygen and hydrogen gas, though primarily reckoned a simple or elementary substance. "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 hydrogen, 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."

2331. Water is absolutely necessary to the economy of vegetation in its elastic and fluid states; 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 one twelfth, and its contraction of bulk during a thaw, tend to pulverise the soil, to separate its parts from each other, and to make it more permeable to the influence of the air.

CHAP. IV.

Of the Agency of the Atmosphere in Vegetation.

2332. The aerial medium which envelopes the earth may be studied chemically and physically the first study respects the elements of which the atmosphere is composed; and the second their action in a state of combination, and as influenced by various causes, or those phenomena which constitute the weather.

SECT. I. Of the Elements of the Atmosphere.

2333. Water, carbonic acid gas, oxygen, and azote, are the principal substances composing the atmosphere; but more minute enquiries respecting their nature and agencies are necessary to afford correct views of its uses in vegetation.

2334. That water exists in the atmosphere is easily proved. If some of the salt, called muriate of lime, which has been just heated red, be exposed to the air, even in the driest and coldest weather, it will increase 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. That it existed in the air in an invisible and elastic form, is proved by the circumstances, that if a given quantity of air be exposed to the salt, its volume and weight will diminish, provided the experiment be correctly made.

2335. 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 part in volume, or in weight. It is the condensation 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.

2336. The power of different substances to absorb aqueous vapour from the atmosphere by cohesive attraction has been already referred to. The leaves of living plants appear to act upon this vapour in its elastic 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 intense 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.

2337. The existence of carbonic acid gas in the atmosphere is proved by the following process if a solution of lime and water be exposed to the air, a pellicle will speedily form upon it, and a solid matter will gradually fall to the bottom of the water, and in a certain time the water will become tasteless; this is owing to the combination of the lime which was dissolved in the water with carbonic acid gas, which existed in the atmosphere, as may be proved by collecting the film and the solid matter, and igniting them strongly in a little tube of platina or iron; they will give out carbonic acid gas, and will become quicklime, which, added to the same water, will again bring it to the state of lime-water.

2338. The quantity of carbonic acid gas in the atmosphere is very small. It is not easy to determine it with precision, and it must differ in different situations; but where there is a free circulation of air, it is probably never more than one 500th, nor less than one 800th, of the volume of air. Carbonic acid gas is nearly one third 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 chemical 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 attraction, 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 proportion, in the air brought down from great heights in the atmosphere by aeronautic adventurers.

2339. The principal consumption of the carbonic acid in the atmosphere seems to be in affording nourishment to plants; and some of them appear to be supplied with carbon chiefly from this source.

2340. The formation of carbonic acid gas takes place during fermentation, combustion, putrefaction, respiration, and a number of operations taking place upon the surface of the earth; and there is no other extensive operation known in nature, by which it can be destroyed but by vegetation.

2341. Orygen and azote are the remaining constituents of the atmosphere. After a given portion of common air has been deprived of aqueous vapour and carbonic acid gas, it appears little altered in its properties; it remains a compound of oxygen and azote, which supports combustion and animal life. There are many modes of separating these two gases from each other. A simple one is by burning phosphorus in a confined volume of air; this absorbs the oxygen and leaves the azote; and 100 parts in volume of air, in which phosphorus has been burnt, yield 79 parts of azote; and by mixing this azote with 21 parts of fresh oxygen gas artificially procured, a substance having the original characters of air is produced. To procure pure oxygen from air, quicksilver may be kept heated in it, at about 600°, till it becomes a red powder; this powder, when ignited, will be restored to the state of quicksilver by giving off oxygen.

2342. Oxygen is necessary to some functions of vegetables; but its great importance in nature is its relation to the economy of animals. It is absolutely necessary to their life. Atmospheric air taken into the lungs of animals, or passed in solution in water through the gills of fishes, loses oxygen; and for the oxygen lost, about an equal volume of carbonic acid appears.

2343. The effects of azote in vegetation are not distinctly known. As it is found in some of the products of vegetation, it may be absorbed by certain plants from the atmosphere. It prevents the action of oxygen from being too energetic, and serves as a medium in which the more essential parts of the air act; nor is this circumstance unconformable to the analogy of nature; for the elements most abundant on the solid surface of the globe are not those which are the most essential to the existence of the living beings belonging to it.

2344. The action of the atmosphere on plants differs at different periods of their growth, and varies with the various stages of the developement and decay of their organs. If a bealthy seed be moistened and exposed to air at a temperature not below 45°, it soon germinates, and shoots forth a plume, which rises upwards, and a radicle which descends. If the air be confined, it is found that in the process of germination the oxygen, or a part of it, is absorbed. The azote remains unaltered; no carbonic acid is taken away from the air; on the contrary, some is added. Seeds are incapable of germinating, except when oxygen is present. In the exhausted receiver of the air-pump, in pure azote, or in pure carbonic acid, when moistened they swell, but do not vegetate; and if kept in these gases, lose their living powers, and undergo putrefaction. If a seed be examined before germination, it will be found more or less insipid, at least not sweet; but after germination it is always sweet. Its coagulated mucilage, or starch, is converted into sugar in the process; a substance difficult of solution is changed into one easily soluble; and the sugar carried through the cells or vessels of the cotyledons is the nourishment of the infant plant. The absorption of oxygen by the seed in germination has been compared to its absorption in producing the evolution of fatal life in the egg; but this analogy is only remote. All animals, from the most to the least perfect classes, require a supply of oxygen. From the moment the heart begins to pulsate till it ceases to beat, the aeration of the blood is constant, and the function of respiration invariable: carbonic acid is given off in the process; but the chemical change produced in the blood is unknown; nor is there any reason to suppose the formation of any substance similar to sugar. It is evident, that in all cases of semination, the seeds should be sown so as to be fully exposed to the influence of the air; and one cause of the unproductiveness of cold clayey adhesive soils is, that the seed is coated with matter impermeable to air. In sandy soils the earth is always sufficiently penetrable by the atmosphere; but in clayey soils there can scarcely be too great a mechanical division of parts. Any seed not fully supplied with air, always produces a weak and diseased plant. We have already seen that carbon is added to plants from the air by the process of vegetation in sunshine; and oxygen is added to the atmosphere at the same time. It is worthy of remark that the

absence of light is necessary to the formation of sugar in the germination of seeds; and its presence to the production of sugar in fruits. The following is the late Dr. Murray's ingenious explanation of these remarkable facts. The seed consists chiefly of farinaceous matter, which requires oxygen to convert it into sugar. Now living vegetables appear to absorb oxygen in the dark: unripe fruits usually contain an acid, that is, have an excess of oxygen; and light is favourable to the evolution of oxygen from living plants. (T.) 2345. Those changes in the atmosphere which constitute the most important meteorological phenomena may be classed under five distinct heads; the alterations that occur in the weight of the atmosphere; those that take place in its temperature; the changes produced in its quantity by evaporation and rain; the excessive agitation to which it is frequently subject; and the phenomena arising from electric and other causes, which at particular times occasion or attend the precipitations and agitations alluded to. All the above phenomena prove to demonstration that constant changes take place, the consequences of new combinations and decompositions rapidly following each other.

2346. With respect to the changes in the weight of the atmosphere, it is generally known that the instrument called the barometer shows the weight of a body of air immediately above it, extending to the extreme boundary of the atmosphere, and the base of which is equal to that of the mercury contained within it. As the level of the sea is the lowest point of observation, the column of air over a barometer placed at that level is the longest that can be obtained.

2347. The variations of the barometer between the tropics are very trifling; they increase gradually as the latitude advances towards the poles, till in the end it amounts to two or three inches. The following Table will explain this gradual increase:

Latitude.

Places,

0° 0 22 23 33 55 40 55

Peru
Calcutta
Cape Town

Naples

51

8

Dover

53 13

53

23

59 56

Middlewich
Liverpool
Petersburgh

Range of the Barometer.

Greatest.

00112323

Annual.

11011112

89

0 77

1

00

2

47

00

45

2 77

80

96

2348. The range of the barometer is considerably less in North America than in the corresponding latitudes of Europe, particularly in Virginia, where it never exceeds 1'1. The range is more considerable at the level of the sea than on mountains; and in the same degree of latitude it is in the inverse ratio of the height of the place above the level of the sea. Cotte composed a table, which has been published in the Journal de Physique, from which it appears extremely probable, that the barometer has an invariable tendency to rise between the morning and the evening, and that this impulse is most considerable from two in the afternoon till nine at night, when the greatest elevation is accomplished; but the elevation at nine differs from that at two by four twelfths, while that of two varies from the elevation of the morning only by one twelfth, and that in particular climates the greatest elevation is at two o'clock. The observations of Cotte confirm those of Luke Howard; and from them it is concluded, that the barometer is influenced by some depressing cause at new and full moon, and that some other makes it rise at the quarters. This coincidence is most considerable in fair and calm weather; the depression in the interval between the quarters and conjunctions amounts to one tenth of an inch, and the rise from the conjunctions to the quarters is to the same amount. The range of this instrument is found to be greater in winter than in summer; for instance, the mean at York, during the months from October to March inclusive, in the year 1774, was 142, and in the six summer months 1016.

2349. The more serene and settled the weather, the higher the barometer ranges: calm weather, with a tendency to rain, depresses it; high winds have a similar effect on it; and the greatest elevation occurs with easterly and northerly winds; but the south produces a directly contrary effect.

2350. The variations in the temperature of the air in any particular place, exclusive of the differences of seasons and climates, are very considerable. These changes cannot be produced by heat derived from the sun, as its rays concentrated have no kind of effect on air; these, however, heat the surface of our globe, from which heat is communicated to the immediate atmosphere; it is through this fact that the temperature is highest where the place is so situated as to receive with most effect the rays of the sun, and that it varies in each region with the season; it is also the cause why it decreases in proportion to the height of the air above the surface of the earth. The most perpendicular rays falling on the globe at the equator, there its heat is the greatest, and that heat decreases gradually to the poles, of course the temperature of the air is in exact unison; from this it appears that the air acquires the greatest degree of warmth at the equator, whence it becomes insensibly cooler till we arrive at the poles; in the same manner the air immediately above the equator cools gradually. Though the temperature sinks as it approaches the pole, and is highest at the equator, yet as it varies continually with the seasons, it is impossible to form an accurate idea of the progression without forming a mean temperature for a year, from that of the temperature of every degree of latitude for every day of the year, which may be accomplished by adding together the whole of the observations and dividing by their number, when the quotient will be the mean temperature for the year. The "diminution," says Dr. Thomson, "from the pole to the equator takes place in arithmetical progression; or to speak more properly, the annual temperature of all the latitudes are arithmetical means between the mean annual temperature of the equator and the pole; and, as far as heat depends on the action of solar rays, that of each month is as the mean altitude of the sun, or rather as the sine of the sun's altitude. Later observations, however, have shown that all the formulæ for calculating the mean temperatures of different latitudes, which are founded on Mayer's

Empirical Equation, though tolerably accurate in the Northern Atlantic Ocean, to latitude 60°, are totally irreconcileable with observations in very high latitudes; and on the meridians, from 70° to 90° W. and E. of London. The results of late arctic voyages, and of Russian travels, have been satisfactorily shewn, by Dr. Brewster (Edin. Phil. Tr.), to prove the existence of two meridians of greatest cold in the northern hemisphere; and the mean temperature of particular countries varies, not only according to the parallels of latitude, but also according to their proximity to these two cold meridians. (T.)

2351. Inconsiderable seas, in temperate and cold climates, are colder in winter and warmer in summer than the main ocean, as they are necessarily under the influence of natural operations from the land. Thus the Gulf of Bothnia is generally frozen in winter, but the water is sometimes heated in the summer to 70°, a state which the opposite part of the Atlantic never acquires; the German Sea is five degrees warmer in summer than the Atlantic, and more than three colder in winter; the Mediterranean is almost throughout warmer both in winter and summer, which therefore causes the Atlantic to flow into it; and the Black Sea, being colder than the Mediterranean, flows into the latter.

2352. The eastern parts of North America, as it appears from meteorological tables, have a much colder air than the opposite European coast, and fall short of the standard by about ten or twelve degrees, There are several causes which produce this considerable difference. The greatest elevation in North America is between the 40th and 50th degree of north latitude, and the 100th and 110th of longitude west from London; and there the most considerable rivers have their origin. The height alone will partly explain why this tract is colder than it would otherwise be; but there are other causes, and those are most extensive forests, and large swamps and morasses, all of which exclude heat from the earth, and consequently prevent it from ameliorating the rigour of winter. Many extensive lakes lie to the east, and Hudson's Bay more to the north; a chain of mountains extends on the south of the latter, and those equally prevent the accumulation of heat; besides, this bay is bounded on the east by the mountainous country of Labrador, and has many islands; from all which circumstances arise the lowness of the temperature, and the piercing cold of the north-west winds. The annual decrease of the forests for the purpose of clearing the ground, and the consumption for building and fuel, is supposed to have occasioned a considerable decrease of cold in the winter; and if this should be the result, much will yet be done towards bringing the temperature of the European and American continents to something like a level.

2353. Continents have a colder atmosphere than islands situated in the same degree of latitude; and countries lying to the windward of the superior classes of mountains, or forests, are warmer than those which are to the leeward. Earth always possessing a certain degree of moisture, has a greater capacity to receive and retain heat than sand or stones, the latter therefore are heated and cooled with more rapidity: it is from this circumstance that the intense heats of Africa and Arabia, and the cold of Terra del Fuego, are derived. The temperature of growing vegetables changes very gradually; but there is a considerable evaporation from them if those exist in great numbers, and congregated, or in forests, their foliage preventing the rays of the sun from reaching the earth, it is perfectly natural that the immediate atmosphere must be greatly affected by the ascent of chilled vapours.

2354. Our next object is the ascent and descent of water: the principal appearances of this element are vapour, clouds, dew, rain, frost, hail, snow, and ice.

2355. Vapour is water rarefied by heat, in consequence of which, becoming lighter than the atmosphere, it is raised considerably above the surface of the earth, and afterwards by a partial condensation forms clouds. It differs from exhalation, which is properly a dispersion of dry particles from a body. When water is heated to 212° it boils, and is rapidly converted into steam; and the same change takes place in much lower temperatures; but in that case the evaporation is slower, and the elasticity of the steam is smaller. As a very considerable proportion of the earth's surface is covered with water, and as this water is constantly evaporating and mixing with the atmosphere in the state of vapour, a precise determination of the rate of evaporation must be of very great importance in meteorology. Evaporation is confined entirely to the surface of the water; hence it is, in all cases, proportional to the surface of the water exposed to the atmosphere. Much more vapour of course rises in maritime countries or those interspersed with lakes, than in inland countries. Much more vapour rises during hot weather than during cold: hence the quantity evaporated depends in some measure upon temperature. The quantity of vapour which rises from water, even when the temperature is the same, varies according to circumstances. It is least of all in calm weather, greater when a breeze blows, and greatest of all with a strong wind. From experiments, it appears, that the quantity of vapour raised annually at Manchester is equal to about 25 inches of rain. If to this we add five inches for the dew, with Dalton, it will make the annual evaporation 30 inches. Now, if we consider the situation of England, and the greater quantity of vapour raised from water, it will not surely be considered as too great an allowance, if we estimate the mean annual evaporation over the whole surface of the globe at 35 inches.

2356. A cloud is a mass of vapour, more or less opaque, formed and sustained at considerable height in the atmosphere, probably by the joint agencies of heat and