This colorful gas which covers the surface of the sun, is called the _chromosphere_ or color-sphere on account of the vivid color which it acquires from the hydrogen and calcium of which it is largely composed. The colossal flames which rise above its surface are called _prominences_. Some of the prominences shoot up with a velocity ranging from 300 to 600 miles a second, sometimes to a height of 200,000 miles, and even higher. The largest so far recorded extended upward for 500,000 miles. Others extend horizontally for a distance of over 300,000 miles, racing forward at the rate of a thousand miles a minute, although sometimes they remain in an unchanged position above the surface of the sun for days at a time. These flames seem tremendous compared to our terrestrial standards, but they would not seem so large on the globe of the sun which is 3,000,000 miles in circumference. [Figure 134: HYDROGEN FLAMES ON THE SURFACE OF THE SUN. Photographed with the aid of the spectroheliograph at the Mount Wilson Observatory.] It seems strange to think that the "tranquil stars" must also be seething in such wild commotion,--blazing orbs torn by terrific tornadoes of fire. It is fortunate that we cannot hear the terrible roar as well as see the light of their conflagrations. Since sound waves do not travel through space like the waves of light and heat, the tumult on even our nearest star could never reach us; yet sound is such a laggard traveler compared with fleet-winged light, that light could cover the distance of 93,000,000 miles in less than 8 minutes while sound would require over 14 years to travel over the same space. Thus, even if we could hear the voices of these explosions, they would never coincide with the scenes enacted. The bright yellow surface of the sun which every one sees, is called the _photosphere_ or light-sphere. If strongly magnified through a telescope, this surface is seen to have a decided "mottled" appearance, like a layer of little clouds with dark spaces between, spread over the whole face of the sun. These are now known to be masses of calcium gas. Practically all that is known of the physical constitution of the sun has been discovered through the study of the fine, dark lines in the solar spectrum. Through the aid of the spectroscope, many terrestrial elements have been discovered, among these being calcium, sodium, magnesium, potassium, aluminum, silver, iron, nickel, copper, zinc, tin, lead, hydrogen, oxygen and carbon. All these elements are either in the form of permanent gases or metallic vapors.