LITMIR.BIZ

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relations with all the physical phenomena of our globe. Whence proceed the winds? From the action of the sun. The sun heats the different portions of the earth very unequally, bestowing much more warmth on the tropical and equatorial regions than on the other latitudes, which he leaves exposed to cold. On each point of the earth which is struck by the rays of the sun, the layers of air near the ground are dilated and raised, and immediately replaced by colder layers from the temperate regions. Thus the periodical winds are produced. Across the hemispheres two great aërial currents are perpetually blowing, going from the equator to each of the poles; one, the upper current, towards the north-east in the northern hemisphere, and towards the south-east in the southern hemisphere; the other, the lower current, in a contrary direction.

      The movement of the earth gives rise to other regular winds. The action of heat and of evaporation, added to the unequal distribution of the continents and the seas, produce others, which are irregular. Thus, for example, in the great valleys of the Alps, as in those of the Cordilleras, the warmth of the air regulates the afflux of the cold air of the mountains, and brings on tumultuous winds, and, in fact, hurricanes.

      The sea breezes arise from the difference in the temperature of the shore during the day and the night. By day, the sun has warmed the shore and produced a considerable dilatation of the air. When the sun quits the horizon, this hot air is replaced by cool currents from the inland. The same phenomenon is reversed in the morning, when the sun returns; the shore is warmed, the hot air rises, and is replaced by the colder air of the sea, which then goes inland. Thus, the evening breeze comes from landward, and the morning breeze from seaward.

      We see, therefore, that the great atmospheric movements which we call the winds, are due to the successive appearances and disappearances of the sun, as are also the lesser movements which we call breezes. The position of the sun, constantly varying according to the period of the year, and the hour of the day, explains the inequality and the continuous existence of the aërial current.

      The general cause of the winds which preserve the homogeneity of the air in all the terrestrial regions, is the heat of the sun dilating the atmospheric air; its absence, on the other hand, causes that gaseous mass to contract.

      The watering of the globe, that is to say the rain, an element indispensable to the exercise of life, is another consequence of solar heat. The waters of the seas, the rivers, and the lakes, those which steep the soil, or are exhaled from vegetable matter, are gradually transformed into vapour by the action of the sun's heat, and form clouds and invisible vapour. When the sun has quitted the horizon, these vapours grow cold in the bosom of the atmosphere in which they floated, and fall down upon the earth again in the form of dew, of fog, and of rain.

      When the cooling of the watery vapour in the bosom of the atmosphere is more intense, instead of rain we have snow, that is to say, a fall of congealed water. It is chiefly on the summit of mountains that snow falls and accumulates, because the temperature of elevated places is always cold. In very great altitudes the snow, remaining for long periods on the tops of the mountains, passes into an intermediate condition, between snow and pure ice, and ends by forming those great expanses of congealed water which are called glaciers. During the hot seasons the glaciers melt by degrees; the water resulting from this melting process, flows down the slopes of the mountains into the valleys, and gives rise to springs, rivers, and streams. These streams and rivers run into the ocean, from which they are again evaporated by the action of solar heat, and reconstitute clouds and invisible vapour.

      Thus is established and maintained that incessant circulation of the waters which lie on the surface of the earth, their continual exchange with the aërial masses, whose effect is to water the globe, a phenomenon necessary to the exercise of the functions of organized beings.

      The regular currents which furrow the waters of the ocean are also the result of the action of solar heat. From the poles to the equator the waters of the sea are unequally heated, and this absence of equilibrium in the temperature of the sea occasions a regular furrow, or line from the poles to the equator, resulting from the displacement of the waters, the cold waves rushing in to replace the hot. The unequal evaporation caused by the unequal distribution of heat at the equator and the poles, concurs to produce a similar result, by augmenting the degree of saltness at the equator, without augmenting it at the poles, occasioning a certain difference in density, and finally displacement for want of equilibrium. The currents of the sea are thus entirely produced by the action of the sun.

      We see, therefore, that the winds, the watering of the globe, and the currents of the sea are the consequence of solar heat.

      The movement of the magnet is another physical result of the action of the sun, if it be true, as Ampère says, that the magnetic currents which traverse the terrestrial globe are nothing but thermo-electric currents engendered by the unequal distribution of heat on the surface of the globe.

      In addition to being the agent of powerful physical forces, the sun is a valuable agent of chemical forces,—indeed, this is the greatest part which he plays in the phenomena of nature. The light and heat of the sun produce the most important chemical actions on the earth's surface; those on which the exercise of vegetable and animal functions depend. If the sun did not exist, life would be banished from the terrestrial globe. Life is the child of the sun, as I shall endeavour to prove to you.

      The operations of photography serve to make us understand how it is that the sun presides over chemical action in the vegetable world. What is photography? What does that curious phenomenon which fixes a drawing formed by light upon a sheet of paper, consist of? A paper steeped in chloride or iodide of silver is placed in the focus of the lens of a dark camera, and the image formed by the lens is made to fall upon paper sprinkled with water. The portions of the picture not exposed to light produce no effect upon the salt of silver, which is incorporated with the paper, but the portions exposed to light decompose the salt of silver, and turn it black, or dark violet colour. On withdrawing this paper from the apparatus, where the operations have been carried on in darkness, we have a drawing which reproduces, in black, the luminous image formed by the lens. By certain means this image, solely produced by the chemical action of light, is rendered fixed and unalterable.

      All the salts of silver thus exposed to light undergo an analogous decomposition. Nor are they the only salts which light modifies. Compounds of gold, platinum, and cobalt, properly prepared, may also be altered under the influence of direct or indirect rays, when exposed to the sun, or to his diffused light.

      The light of the sun possesses the power of bringing about the combination of several other bodies. This is the case with hydrogen and chloric gas. If you mix equal parts of chloric gas and hydrogen in a bottle, and expose the mixture to the sun, an immediate combination will take place between the two gases, and chlorohydric acid gas will be formed. The combination will take place with so much force that it will be attended by a considerable escape of heat. If you throw the bottle containing the mixture up into the air, towards a space where the sun is shining, the bottle will break before it falls, with a violent explosion, at the moment of its contact with the light.

      We might multiply examples of the chemical action produced by light only on substances belonging to the mineral kingdom, but it is sufficient for our purpose to say that the chemical action of light is still more powerful and more general in the vegetable than in the inorganic realm. This is a phenomenon of such importance that it is impossible to believe it otherwise than a premeditated design of nature.

      One of the most fruitful discoveries of modern science is the recognition of the fact, that the respiration of plants depends upon the presence and the direct action of light, that is to say, that the decomposition of the carbonic acid which circulates in the tissue of vegetables, and which has been breathed up from the soil by the roots, takes place only when the plants are exposed to the sun. The labours of Priestley, Charles Bonnet, Ingenhouz and Sennebier, have taught us that the decomposition of carbolic acid into carbon, which remains fixed in the tissue of the plant, and into oxygen, which disengages itself from it, can take place only under the direct or indirect influence of the sun's rays. Our readers may easily convince themselves of this fact. Place a handful of green leaves in a glass full of water, and expose the glass to the sun. At the close of the day the upper portion of the glass will be filled with gas, which is nothing but pure oxygen, the result of the breathing of the leaves.

      All

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