Saturation

“ The saturation point rises with civilization just as the saturation point of air for water rises with the temperature. Cultivated land is said to produce 1,600 times as much food as an equal area of hunting land. ”

“ Such atoms are designated “unsaturated.” Saturation has been described by Russell [115] as follows:—“For the strong lines ... the absorption in the reversing layer is so great that a large increase in the number of absorbing atoms present alters the strength of the line very little. For the weak components ... absorption under ordinary conditions is incomplete, and the strengthening (in the spectra of sunspots) is noteworthy”—an increase in the amount of available material produces an increase in the strength of the line. ”

“ The moisture sensation of the air is its humidity. Before saturation is reached, the moisture is in the form of vapor; at the point of saturation it may appear in the air as fog, or cloud; when the ground temperature reaches the point of saturation condensation takes place in the form of dew ”

“ He regarded natural or white light as the simplest type of disturbance, being constituted by a simple and uniform pulse at right angles to the direction of propagation, and inferred that colour is generated by the distortion to which this disturbance is subjected in the process of refraction, "The Ray," [30] he says, "is dispersed, split, and opened by its Refraction at the Superficies of a second medium, and from a line is opened into a diverging Superficies, and so obliquated, whereby the appearances of Colours are produced." ”

“ In a few seconds, however, I was looking down upon its upper surface, reflecting from a thousand broken masses of vapour at different levels, from cavities and hillocks of mist, the light of the sun; white beams mixed with innumerable rays of all colours in a confusion, of indescribable brilliancy. I presume that the total obscuration of everything outside the cloud during my passage through it was due to its extent and not to its density, since at that height it could not have been otherwise than exceedingly light and diffuse. ”

“ The minute particles of dust in the atmosphere arrest the sun’s rays and scatter them in all directions; they are so small, however, that they cannot reflect and scatter all; their power is limited to [Pg 64] the scattering of the rays at the blue end of the spectrum, while the red rays pass on unarrested. The display of the colours of the blue end are found in numberless shades, from the full deep blue in the zenith to the greenish-blue near the horizon. ”

“ The whole body of vibrations caused by the sun is termed radiation, or, more commonly, rays; and consists of sets of waves which vary considerably in their dimensions and rate of recurrence, but of which the middle portion only is capable of exciting in us sensations of light and colour. Beginning with the largest waves, which recur at the longest intervals, we have first those which produce heat-sensations only; as they get smaller and recur quicker, we perceive a dull red colour ”

“ The white or yellow light, known to us as sunlight, is produced by the ground-glass globe of air, and it follows readily enough that its intensity is absolutely dependent upon the density of the atmosphere—the thickness of the globe. The cause for the thickening of the aërial envelope lies in the particles of dust, soot, smoke, salt, and vapor which are found floating in larger or smaller proportions in all atmospheres. ”

“ It will thus be seen that in inducing phototropic curvature, the heat rays in sunlight play as important a part as the more refrangible rays of the spectrum.
SUMMARY.
The effects of rise of temperature and of radiation are antagonistic to each other.
Under unilateral action of thermal radiation a positive curvature is induced by the retardation of growth at the proximal, and acceleration of growth at the distal side of the organ.
There is a complete recovery on the cessation of stimulus of moderate intensity and short duration. ”

“ The small particles of water and dust—and when we say small we mean small when measured on the same scale as we measure the lengths of waves of light—differentiate between waves of different lengths, and scatter the blue rays more than the green, and the green than the red; consequently what the sun lacks in blue and green is to be found in the light of the sky. ”

“ But farther: these questions of volatility, and visibility, and hue, are all complicated with those of shape. How is a cloud outlined? Granted whatever you choose to ask, concerning its material, or its aspect, its loftiness and luminousness,—how of its limitation? What hews it into a heap, or spins it into a web? Cold is usually shapeless, I suppose, extending over large spaces equally, or with gradual diminution. You cannot have, in the open air, angles, and wedges, and coils, and cliffs of cold. ”

“ If this be a true account of the solar heat, for the support of which I appeal to my experiments, it remains only for us to admit that such of the rays of the sun as have the refrangibility of those which are contained in the prismatic spectrum, by the construction of the organs of sight, are admitted under the appearance of light and colors, and that the rest, being stopped in the coats and humors of the eye, act on them, as they are known to do on all the other parts of our body, by occasioning a sensation of heat. ”

“ The simple theory of the dispersion of light by small particles suffices to explain not only the blue of the zenith, but the comparative absence of small wave-lengths from the direct solar rays, and the brilliant orange and red coloration of the setting sun and of the clouds illuminated by his rays. The hyposulphite experiment here again affords an excellent illustration. But we must not expect a simple theory to cover all the facts. ”

“ For the purposes of this paper therefore the reader will kindly assume refraction to stand for that intimate effect of one mass of color or value upon its adjoining mass which results in the "lost-edge," and a general diffusion of tone, thus giving to pictures their atmospheric quality.
Now refraction is only in a very limited sense an objective fact. It is mainly a visual fact whose operation is due to the imperfect construction of the lens of the human eye. The scientific fact is that the edges of things are sharp and hard as a rule. ”

“ So, also, when white light falls upon what we call a red surface, the yellow and blue rays are stifled or absorbed, leaving the red only to be reflected. Now, when we consider the various ways in which this absorption may take place; one or two, or all of the coloured rays being absorbed in every possible proportion, it is easy to form some idea of the manner by which the innumerable tints of the sky are produced. ”

“ TROPIC EQUILIBRIUM UNDER VARYING INTENSITIES OF STIMULUS.
The following experiment will show that the position of tropic equilibrium is not fixed but subject to variation under changes of effective stimulation.
Fig. 185.—Effect of variation of intensity of light on phototropic equilibrium. Increase of intensity of light from L to L' produces an increased positive curvature and a new state of balance. Diminished intensity of light l brings about a new balance at a lower level. The cessation of light (l within a circle) restores the normal position of the organ. ”

“ What is meant by negative is, that the bright bands in the spectrum of any substance, when diffused in the flame of a lamp, become black in the solar spectrum. Suppose that, when viewmg the solar spectrum, the light of the solid or liquid photosphere of the sun were suddenly extinguished, the gaseous atmosphere still glowing, the colours which we at present see would be at once extinguished, and all the dark lines would become bright bands with their appropriate colours. ”

“ The excessive heating of the plain by the vertical rays of the sun causes the variations of refraction,—occasioned by the effects of radiation, of the ascending current, and of the contact of strata of air of unequal density,—to continue through the entire night. ”

“ Speaking generally, it may be affirmed that the radiations of obscure heat are more largely absorbed by the gases and vapors of our atmosphere than those of luminous heat, and the great differences in the mere luminosity of the spots, penumbra, and photosphere justify the assumption that the radiations of a sun-spot will (to use the expressive simile of Tyndall) lose far more by atmospheric sifting than will those from the photosphere. ”

“ Beyond our atmosphere the sun would appear a rayless orb of fire in the midst of a black concave. The equal diffusion of its light on earth is owing to the refraction of the rays by the atmosphere, and their reflection from other bodies. Light consists either of vibrations propagated through a subtle medium, or of numerous minute particles repelled in all directions from the luminous body. ”

“ As the temperature is further increased, the red light increases, and yellow, green and blue rays are successively thrown off. On raising the temperature to a still higher point, radiations of a wave-length shorter even than violet light are produced, to which the eye is insensitive, but which act strongly on certain chemical substances ”

“ Come down below the color vibrations, and we shall find that those which are too infrequent to be visible, manifest as heat. Naturally there will be as many different kinds of heat as tints of color, because there is as great a range of numbers of vibration. ”

“ The heat in the spectrum evaporated the spirit first on those parts of the paper where it fell with greatest intensity, thereby restoring their white colour, and he thus discovered that the heat increases uniformly and gradually throughout the luminous spectrum, and that it comes to a maximum and forms a spot at a considerable distance beyond the extreme red. ”

“ The capacity of a body to transmit light without absorbing it and becoming luminous is called transparency. Air freed of dust motes is almost perfectly transparent. In this state it is said to be optically pure. But the ordinary air of nature, with its moisture and dust, absorbs most of the blue wave lengths and also many of the longer ones of the other colors of the spectrum. ”

“ In other words, scientists think that when surface areas darken and expand, relatively more energy from the sun is being absorbed by the surface, which causes temperatures to rise near the surface. This, in turn, produces a less stable atmosphere generating more turbulent eddies and whirling dust devils. The more dust that is redistributed to bright surfaces, the more surfaces darken and expand, which causes more sunlight to be absorbed, increased temperatures, and less stable the atmosphere, say scientists. ”

“ The pure atmosphere is a perfectly transparent medium, which neither radiates, absorbs, nor conducts caloric, but transmits the rays of the sun to us without in any way 87 diminishing their intensity. The air is therefore not more heated, by the sun’s rays passing through it, than diamond, glass, water, or any other transparent medium. ”

“ It will readily occur to the reader, that when the lower part of the atmosphere is very much rarified, so that its density diminishes rapidly to the ground, it is possible for a ray of light tending downwards, to be reflected, as it is in glass, or water, when the angle of incidence exceeds a certain value. ”

“ But we cannot conceive of light suddenly losing its phototropic effect by an increase of intensity. The experiment just described will offer full explanation for this apparent anomaly. Feeble or moderate stimulus remains, as we have seen, localised, hence the contraction of the proximal side gives rise to positive curvature. But the intense excitation caused by sunlight would be transmitted to the distal side and thus bring about neutralisation. ”

“ This scattering action of the atmosphere on the solar light is greatly increased by the irregularity of temperature caused by the sun, which throws the atmosphere into a constant state of undulation; and by thus bringing together masses of air of different temperatures, produces partial reflections and refractions at their common boundaries, by which means much light is turned aside from a direct course, and diverted to the purposes of general illumination. ”

“ We have said that all bodies undergo a chemical disturbance when exposed to the solar rays, but it must not be supposed that this disturbance always manifests itself in a blackening, as in the case of horn silver. If a polished plate of metal, of glass, of marble, or even a polished surface of wood, be in part exposed to the influence of sunshine, it will, when breathed upon, exhibit the fact that a disturbance of some kind has taken place upon the portions illuminated, whereas no change can be detected upon the parts kept in the dark. ”

“ Hence, if two lights of unequal illuminating powers shine upon the same surface at equal obliquities, and an opake body be interposed between them and the illuminated surface, the two shadows produced, must differ in blackness or intensity in the same degree. ”

“ That objects, being near the mirage, as trees, animals, men, &c., are seen double, can also be explained by the following law of reflection: When two strata of air, one of common middle temperature and density, and the other hotter, meet together, an observer, standing also in a common temperature and looking at an object near where the two strata meet, will see that object double, directly in the stratum of common air, in which he stands himself, and indirectly by reflected light in the hotter stratum. ”