2. “Haroun-al-Raschid,” Aaron the Just. (765-809.) The caliph who raised Bagdad to its greatest splendor, and whose reign was looked upon as the golden era of the Mohammedan nation. He reigned twenty-three years and performed the pilgrimage to Mecca nine times. He is famous as the hero of the Arabian tales. Tennyson wrote of him in his “Recollections of Arabian Nights.” 3. “Ex post facto.” A Latin expression, meaning an after act or thing done afterward. An _ex post facto_ law is a law enacted after the commission of a crime, for the purpose of being enforced upon the person having committed the crime, who could not be held a criminal, or at least a criminal in the same degree, until after the enaction of the law. All such laws are forbidden by the constitution of the United States. 4. Transcriber’s Note: This note (presumably “Hypolais.”) was omitted in the original. _Hippolais_ is a scientific genus of tree warblers. CHEMISTRY. [ERRATA.—A few typographical errors in articles of this series have escaped correction. Page 254, change “300,000,000” to 3,000,000; for “alcohol,” thirty-third line, page 325, substitute paraffine; same page, eighth line, second column, use not for “but;” in “experiment,” same column, use heat for “sensation,” and in next to the last line of the article change “topics” to optics.—_Prof. J. T. Edwards._] 1. “Geysers,” gīˈsers. Intermittent hot springs, found in different parts of the world. Those of Iceland are the best known. More than one hundred of these springs are there found, within a space of two miles. The geysers of the Yellowstone are the most wonderful ever discovered. The country lying between latitude 43° and 47° north, and longitude 110° and 114° west, is dotted with groups of springs. Some of them, when in action, send up columns of water to a height of 200 feet. 2. “Hugh Miller.” (1802-1856.) A British geologist. He was by trade a stone mason, but he devoted all his leisure hours to study. Soon “detecting the wonders of the fossil world” in the quarries in which he worked, he made them the special subject of his thought, and soon became an eminent geologist. He published many works, most of them bearing on this subject. He worked so incessantly, taking little sleep or exercise, that his mind was on the verge of giving way. Realizing this with terror, he took his own life. A note left for his wife read as follows: “A fearful dream rises upon me. I can not bear the horrible thought.” The old red sandstone is the name given to the rock in Great Britain formed in the Devonian age, or age of fishes. Its thickness is in some parts 8,000 to 10,000 feet. It includes sandstone, marlytes of red and other colors, and some limestone. The sandstone of the Triassic period, which includes the latest formations of the earth’s crust, is also characterized by fossils, and is often red in color; hence the name, new red sandstone, has been applied to it.