radiator and a bad conductor of heat, will therefore be covered with a very abundant dew. Thus glass becomes wet sooner than the metals; organised bodies are wetted more quickly than glass, especially when they are in small fragments; because, as the heat passes with difficulty from one to the other, that which is lost is not replaced by that which is transmitted from the interior to the surface of the body. Thus locks of wool are very well suited to these experiments, and become covered with a very abundant dew. The moister the air is, all other things being equal, the more considerable is the quantity of dew that falls in a given time. Thus, it is entirely wanting in arid deserts, notwithstanding the intensity of nocturnal radiation. In our countries, nights with abundant dews may be considered as foretelling rain; for they prove that the air contains a great quantity of the vapour of water, and that it is near the point of saturation.* White frost is produced under the same circumstances as dew. While, at a few yards above the earth, the air is several degrees above the freezing-point, the soil is cooled by radiation, and the vapour is congealed in the form of beautiful crystals. This cooling is very hurtful to vegetation; and, during the serene nights of spring, the products of the kitchen-garden are frequently killed by the cold. Here again all circumstances that oppose radiation, prevent the cooling. Vegetables sheltered by trees suffer less than those that are not so protected. A thin covering of cloth or of straw preserves plants; and the vine has often been prevented from being frozen by lighting fires, that give much smoke. In like manner, as under the name of dew are comprised all the drops of water that remain attached to the leaves of plants, so also, under that of hoar-frost, are comprised the aqueous precipitations, which assume the form of snow. These precipitations may be formed in different ways. When south winds succeed to continuous cold, and the thermometer rises almost to freezing-point, vapours are precipitated in the solid form; stone buildings are quite white, and the branches of trees are covered with beautiful crystals. This form of white frost, which is observed in our climates every winter, is very frequent in the polar regions during the winter seasons. The rigging of ships is then adorned with sparkling fringes and regular crystallization, which sailors have called beards. * Vide M. ARAGO's Notice on Dew (Annuaire du Bureau des Longitudes, 1827; pp. 165-198). The ancient chemists thought that they recognised in dewwater celestial principles; it is of great purity, only containing a little more carbonic acid than rain-water. In its contact with vegetables, it becomes charged with organic principles. For a long time, certain dews were supposed to contain foreign substances, and to be hurtful to vegetation. They were called honey-white or miller (Honigthau, Mehlthau, Germ.) They are both sugar secretions, hurtful to vegetables, and to the animals that feed on them. Thus, in the years 1556 and 1669, there was in Switzerland a great murrain; but Scheuchzer, who studied it, suspected then that this substance, that covered the plants, did not fall from the sky. After him, Leche shewed that the aphides, which often collect there in great quantities, are the immediate cause of these pathological secretions. This matter is secreted from two openings, at the posterior part of the animal; if it is not collected either by the bees or the ants, it is dissolved in the dew, and falls on the lower leaves. It is also probable that this sugary matter is due to the decomposition of the vegetable juices, analogous to that by which starch is converted into sugar in the manufacture of beer. ON FOG.-When the vapour of water is precipitated in the atmosphere, the transparency of the air is disturbed; and this aqueous precipitation takes the name of fog, when it is on the surface of the earth, and of cloud, when it remains suspended at a certain height in the atmosphere. So the traveller, who journeys to the summit of a high mountain, complains that the fog intercepts his view; whilst the inhabitant of the plains says that the summit of the said mountain is enveloped in clouds. VESICLES OF FOGS.-Examined by a lens, the fog is composed of small opaque bodies. A close investigation shews that these small bodies are composed of water. Obedient to the laws of universal gravitation, the molecules of water are grouped into the form of spherules, analogous to those of mercury poured into a porcelain saucer, or water at the bottom of a glass smeared with grease. Are these spherules full or empty? On this question meteorologists are divided. The opinion put forth some time ago by Halley, that these spherules are hollow, and that the water only serves as an envelope, appears to have a better foundation than any other. However, it is probable that they are mixed with a great quantity of drops of water; in the sequel, we shall use the terms vesicular vapour, vesicular fog, to designate this particular condition of the vapour of water. The researches of De Saussure and Kratzenstein give great weight to this opinion. Take a cup, filled with a liquid of a deep colour, such as coffee, or Indian ink dissolved in water; warm it, and place it in the sun or in a light place if the air is tranquil, the vapour ascends and soon disappears; if it is observed through a lens, globules of various thicknesses will be seen to ascend from the surface of the liquid. De Saussure adds that the little vesicles that rise differ so much from those that fall, that it is impossible to doubt the former being hollow. The manner in which these bodies conduct themselves with light, is no less favourable to this opinion; they do not present that scintillation which is observed in full drops when exposed to a strong light. Moreover, true rainbows are never observed on clouds, although the spectator, the cloud, and the sun may be often in the relative positions most favourable to the production of the phenomenon; this would not be the case if clouds were composed of drops of water. Kratzenstein made a still more convincing observation, which few authors have taken into consideration. The bubbles formed with soap-water are often ornamented with the most beautiful colours. Colours are also observed on bubbles formed of viscous substances; and they may be studied with the greater facility, as they last a considerable time. A bubble of this kind, placed on black pitch or melted glass, shews, at its upper part, a black or a coloured spot, surrounded by a certain number of coloured rings. These colours are derived from the incident rays being divided into two portions. Some are reflected by the anterior surface; others traverse it, but are partly reflected by the posterior surface. The eye, therefore, receives, almost in the same direction, two classes of reflected rays, some from the anterior, others from the posterior surface. These rays, being variously coloured, react on each other, and are neutralised; but some of the colours of the spectrum remain isolated; and it is not white, but coloured light, which reaches the eye. I shall not examine in this place the causes and the laws of the phenomenon; but I would merely remark that the envelope of the sphere must be very thin, in order that these appearances may be produced, and that they are intimately connected with the thickness of this envelope. In soap-bubbles, these appearances change every moment; because the water which runs on the bubble, and evaporation every moment, vary the thickness of the envelope. In like manner, the colours of a film of mica change every moment, when it is pressed between the fingers, because the thickness of the film of air, which separates the different films, never remains the same. 6 Vide Note e, Appendix No. II. themselves on; they do in full drops true rainbows spectator, the e positions most Tenon; this would drops of water. incing observation, consideration. The be ten ornamented with are also observed on s: and they may hey last a considerable ed on black pitch or t, a black or a coloured mber of coloured rings. e incident rays being die reflected by the anterior re partly reflected by the erefore, receives, almost in of reflected rays, some from sterior surface. These rays, on each other, and are neuours of the spectrum remain te, but coloured light, which not examine in this place the henomenon; but I would merely of the sphere must be very thin, rances may be produced, and that nected with the thickness of this les, these appearances change every water which runs on the bubble, moment, vary the thickness of the anner, the colours of a film of mica when it is pressed between the fingers, of the film of air, which separates the remains the same. Note e, Appendix No. II. To study these optical effects, Newton's process is the best. Take a piece of very level glass, and on it place a convex lens with a long focus; on looking at it, at a certain angle, you will see coloured rings, the centre of which coincides with the point of contact of the two glasses. If the radius of the curvature of the lens were known, the distance of the different points of the lens from the level mirror may be calculated; if, at the same time, the colours are observed, the thickness necessary to produce such or such a colour is deduced. Kratzenstein, having examined in the sun, through a magnifying glass, the vesicles rising from hot water, observed at their surface coloured rings, like those of soap-bubbles; and not only was he convinced that their structure was analogous to soap-bubbles, but he was also able to calculate the thickness of their envelope. De Saussure and Kratzenstein endeavoured to measure under the microscope the diameter of the vesicles, of which the vapour of water is composed. It is, however, difficult to arrive at a positive result; for the true object is to measure vesicles of fog, and not those arising from hot water: fortunately some of the optical phenomena, that are produced when the sun shines through clouds or fogs, furnish us with a means of arriving at this result. I will hereafter describe the process that I adopted, regretting that it has not been more frequently put into practice. I have made a great number of measurements in Central Germany and in Switzerland. I found that the mean diameter of the vesicles of fog is about 0mm,0224; their diameter varies in different seasons, and appears to be smaller in summer; I find, for instance, the following numbers : DIAMETERS OF VESICLES OF FOG IN THE DIFFERENT MONTHS A very regular progression is seen to exist from winter to summer; for the anomalies depend on the insufficient number of existing observations. So, in winter, when the air is very moist, the diameter of the vesicles is twice as great as in summer, when the air is dry; but in the same month this diameter also changes: it obtains its minimum when the weather is very fine; it increases as soon as there is a threatening of rain; and before it falls, it is very unequal in the same cloud, which probably contains a great number of drops of water, mingled with the vesicular vapour. Kratzenstein determines the thickness of the envelope of these vesicles from the coloured ring, which he observed on their surface; it is 0,06. FORMATION OF FOGS. -When fog becomes visible any where, it is because the air is saturated with moisture; then only can the vapour of water be precipitated incessantly for several hours. It is important to insist on this circumstance; for Deluc, and some other philosophers, who have employed imperfect hygrometers, have insisted that the air is often very dry, in regions where fogs are forming. The experiments of De Saussure, however, prove the contrary; and I have convinced myself of the fact on the Alps, and in different parts of Germany. An hygrometer suspended before a window in the centre of a city, undoubtedly cannot indicate the degree of saturation during the times of a fog; but this occurs, because the instrument is warmed by the walls of the building and even this anomaly eventually disappears, when the fog remains for several hours. The circumstances amid which fog forms, are often very different from those which accompany dew. When the latter is deposited, the soil is always colder than the air; when fog occurs, the contrary is the case: the moist soil is warmer than the air; and the vapours that ascend become visible, like those which rise from boiling water, or like the vapour of expired air, which, in winter, condenses the moment it escapes from the mouth. So, in autumn, we frequently see fogs above rivers, the water of which is much warmer than the air before sun-rise.* + However, the water and the soil may be hotter than the Fig. 14 in the Appendix clearly shews the relative range of the temperatures of the air and of rivers in the city of Lyon, situated, as is well known, at the influx of the Rhône and the Saône. The inspection of the three curves shews that, from the 1st of November, the mean temperature of the air becomes lower than that of the rivers; a state of things which ceases about the 1st of March; now that intermediate period is the true season of fogs. The comparison of the three curves, of the epochs of maxima a or ice or Gian |