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ACHAIA , one of the ancient divisions of the Peloponnesus, extending along the Gulf of Corinth. Its inhabitants were the most powerful of the Greeks at the time of the Trojan war. Under the Romans Achaia included the whole Peloponnesus as well as the country across the gulf as far as Thessaly. In modern Greece it is a small province.

, in ancient mythology, a river of the lower world around which hovered the shades of the departed, and across whose waters the ferryman, Charon (ka′ron), piloted those who were permitted to enter the realm of the dead. Acheron was also a general name for Hades.

 was the bravest of the Greeks in the Trojan war, and the hero of the Iliad. His father, Peleus, was a descendant of Zeus, the king of the gods, and the ruler of the Myrmidons, the warlike people of Phthia, in Thessaly. His mother, Thetis, a sea goddess, is said to have dipped him by the heel into the river Styx to make him invulnerable, as she had been forewarned that he was doomed to an early death. For the same reason, after he had been trained in the arts of war and eloquence by Phœnix, and in the healing art by the centaur, Chiron (ki′ron), his mother had him brought up secretly as one of the daughters of the King of Scyros (si′ros). At the outbreak of the Trojan war an oracle declared that Troy could not be taken unless Achilles were present. So Ulysses, the wisest of the Greeks, came to Scyros disguised as a peddler, and spread out his wares before the daughters of the king. Ulysses sounded an alarm, and while the girls ran away the disguised Achilles betrayed himself by seizing a sword and spear from the peddler’s stock. Achilles went to war with fifty-seven ships, and during the first nine years he sacked twenty-three cities around Troy. He quarreled with Agamemnon over a maid, Briseis, whom he loved. When she was taken from him he sulked in his tent, while his countrymen were hard pressed because their bravest warrior, whom the Trojans dreaded, was not there. At last his friend Patroclus, wearing the armor of Achilles, drove the enemy before him, but was slain by Hector, the leader of the Trojans. Achilles, enraged at the death of his friend, went against the Trojans and drove them within their walls. In single combat he killed Hector, whom he dragged three times around the city at his chariot wheels. Here the Iliad ends, but the story is taken up by the Æthiopis, a poem by Arctinus, which tells of the combat of Achilles, first with the Amazon Penthesilea, and next with Memnon. When Memnon fell, Achilles drove back the Trojans to the Scæn gate, where he was killed by an arrow from the bow of Paris, which pierced his vulnerable heel.

, a term used in chemistry to denote a class of substances whose union with an alkali, or other base, forms salts. Strictly speaking, all acids contain hydrogen, and are, in fact, salts of hydrogen. Most of them have the following properties: they can be dissolved in water; they have a sour taste; they turn vegetable blues to red. The most common and useful inorganic or “mineral” acids are sulphuric, nitric and hydrochloric acids, which are manufactured on a very large scale. Among the organic acids are acetic acid, which gives vinegar its sour taste: citric acid, which produces the sourness in lemons; oxalic acid, which is found in sorrel and some other plants, and which in large quantities acts as a poison; malic acid, found in apples and also currants and gooseberries; tartaric acid, found in grapes and used in the manufacture of baking powder; prussic or hydrocyanic acid, a deadly poison, a small quantity of which is found in bitter almonds and in the leaves and stones of peaches. Many hundred acids are known to chemists, the greater part of which are artificial.

, a central province of the Republic of , bounded on the north by the province of Coquimbo, on the south by Santiago and on the southwest by Valparaiso, flanked on the east by the Andes and the Argentine Republic, and on the west by the South Pacific Ocean. Its area is 5,485 square miles, with a population (1910) of 131,331. Its capital is Felipe. On the range of the Andes, within the Argentine boundary, is the extinct volcano of Aconcagua, deemed the loftiest elevation in the New World, with an estimated height of 23,000 feet. The Aconcagua River flows seaward through the province and gives the latter its chief fertility in grain, hemp and a variety of fruits. Copper deposits are found in the province.

, fruit of the oak, a nut once considered an important article of food. The ancients thought eating “oaken mast” gave length of years and strength to man. The Indians of New England and farther south ate the acorns of white oaks of several species. The sweet acorn of the California white oak, Indians of the Pacific Coast bake, shell and grind into a coarse meal from which they make bread. Chinese and Japanese use certain acorns for food. Today in some English villages the people hold to the old “right of pannage,” and in autumn turn their hogs into the royal forests to fatten on the fallen acorns.

. Those phenomena which one detects by the ear are generally studied together under the head of acoustics. But whenever any sound is heard we find that somewhere in the neighborhood there is what we call a