When there have been unusually violent volcanic eruptions, and great quantities of finely divided dust have been thrown into the upper atmosphere, the effect upon the blue and violet rays from the sun is very great. The volcanic dust particles are so large that instead of scattering these rays of shorter wave-length, as do the oxygen and nitrogen in our atmosphere, they reflect them back into space and so decrease the amount of light and heat received from the sun. For this reason the general temperature of the earth is lowered by violent volcanic eruptions. Unusually cold periods, that lasted for months, followed the terrible eruption of Krakatoa in 1883 and of Katmai in 1912. At times when much dust is present in the atmosphere, the sky is a milky white color by day as a result of the reflection of sunlight from the dust particles. Sunrise and sunset colors are then particularly gorgeous, with reds predominating. At such times the blue and violet rays are almost completely shut out, and the red, orange and yellow rays are powerfully diffracted and scattered by the dust particles in the air. The twilight glow that is visible for some time before sunrise or after sunset is, of course, entirely an atmospheric effect caused by the reflection of sunlight to our eyes from the upper atmosphere, upon which the sun shines, while it is, itself, concealed from our view below the horizon. The atmosphere extends in quantities sufficient to produce twilight to an elevation of about sixty miles. When _all_ the rays of which sunlight is composed are reflected in equal proportions we get the impression of white light. Dust and haze in the air reflect all rays strongly and give a whitish color to an otherwise blue sky. Brilliant white clouds appear white, because they are reflecting all rays equally. Clouds or portions of clouds appear black when they are in shade or, at times, by contrast with portions that are more strongly illuminated, or when they are moisture-laden and near the point of saturation, when they are absorbing more light than they reflect. At sunrise and sunset, when the light that falls upon the clouds is richest in red and orange and yellow, clouds reflect these colors to our eyes, and we see the brilliant sunset hues which are more intense the more the air is filled with dust and impurities.