SKUNK CABBAGE (Symplocarkus foetidus or Spathyema foetida), a fleshy herbaceous plant of the arum family, Araceae (q.v.), so called because of its fetid odour and large leaves, native to eastern North America and north-eastern Asia. It grows in swampy places and in very early spring (March or sometimes February) it sends up from thick rootstocks grotesque, swollen, shell-like, purple-brown spathes, each enclosing many small flowers borne in a short thick cluster. These are soon followed by numer ous ovate leaves, i to 3 ft. long, and later by large globular masses of fleshy berries. The similar western skunk cabbage (Lysichiton kamtschatcense) occurs from California to Alaska and also in Siberia.
SKY, the apparent covering of the atmosphere, the overarch ing heaven. (M.Eng. skie, cloud ; O.Eng. skua, shade ; con nected with an Indo-European root sku, cover, whence "scum," Lat. obscurus, dark, etc.) The Colour of the Sky.—It is a matter of common observa tion that the blue of the sky is highly variable, even on days that are free from clouds. The colour usually deepens toward the zenith and also with the elevation of the observer. It is evident that the normal blue is more or less diluted with extraneous white light, having its origin in reflections from the grosser particles of foreign matter with which the air is usually charged. Closely associated with the colour is the polarization of the light from the sky. This takes place in a plane passing through the sun, and attains a maximum about go° therefrom. Under favourable conditions more than half the light is polarized.
As to the origin of the normal blue, very discrepant views have been held. Some writers, even of good reputation, have held that the blue is the true body colour of the air, or of some in gredient in it such as ozone. It is a sufficient answer to remark that on this theory the blue would reach its maximum develop ment in the colour of the setting sun. It should be evident that what we have first to explain is the fact that we receive any light from the sky at all. Were the atmosphere non-existent or absolutely transparent, the sky would necessarily be black. There must be something capable of reflecting light in the wider sense of that term.
A theory that received much support in the past attributed the reflections to thin bubbles of water, similar to soap-bubbles, in which form vapour was supposed to condense. According to it sky blue would be the blue of the first order in Newton's scale of colours. The theory was developed by R. Clausius (Pogg. Ann. vols. 72, 76, 88), who regarded it as meeting the requirements of the case. It must be noticed, however, that the angle of maxi mum polarization would be about 76° instead of 90°.
Apart from the difficulty of seeing how the bubbles could arise, there is a formidable objection, mentioned by E. W. Briicke (Pogg. Ann. 88, 363), that the blue of the sky is a much richer colour than the blue of the first order. Briicke also brought forward an experiment of great importance, in which he showed that gum mastic, precipitated from an alcoholic solution poured into a large quantity of water, scatters light of a blue tint. He remarks that it is impossible to suppose that the particles of mastic are in the form of bubbles. Another point of great im portance is well brought out in the experiments of John Tyndall (Phil. Mag. [4], 137, 388) upon clouds precipitated by the chem ical action of light. Whenever the particles are sufficiently fine, the light emitted laterally is blue in colour and, in a direction perpendicular to the incident beam, is completely plane-polarized.
The dependence of the amount of scattering upon the wave length of the light can be settled in the case of very small par ticles by an application of the method of dimensions. The par ticle acts as a centre for a radiating beam. The amplitude of the light sent out by it at a distance R varies inversely as R ; it is also proportional to the volume of the particle when this is small compared with the wave-length of the light. Thus the ratio of the scattered to the incident intensity varies as ; that is a quan tity whose dimensions are those of the fourth power of a length. The ratio of intensities must, however, be a pure number ; and since the wave length X is the only other linear quantity that can be concerned, the ratio must also depend on the inverse fourth power of X.