body into its parts. Thus, for instance, if we place zinc in dilute sulphuric acid, the zinc will set free hydrogen, which formerly constituted a part of the sulphuric acid. The sulphuric acid has thus been divided into parts. Again, if we confine a mixture of gases in a vessel with porous walls, we find that some of these gases pass through the walls more rapidly than others; and that by collecting those that come through first and those that come through last, we can separate the mixture into parts that are very different from one another. From considerations of this kind we are led to think that matter is made up of particles far transcending in smallness the reach of the most powerful microscope. The smallest mass of any substance in which the properties of the substance still remain is called a molecule. But all substances, except about ninety which are called elements, have been decomposed into other substances having different properties. The smallest mass of each of these elementary substances is defined as an atom. Atom is a Greek word meaning "indivisible;" it acquired its present English signification about the beginning of the 19th century.
An atom is defined as a portion of matter which is indivisible by chemical methods: but there are excellent reasons for thinking that this atom which is indivisible by chemical methods is made up of still smaller parts. One of these reasons is that a single type of atom—say hydrogen—is capable of emitting light of many different wave lengths, just as a piano or an orchestra can emit many different wave lengths of sound. And just as a piano is a complex instrument, so we are led to think that an atom of hydrogen—or indeed an atom of any other substance—is probably a very complex mechanism. A still stronger reason for thinking that the chemical atom is divisible is th fact that Sir Joseph Thomson has recently succeeded in splitting off, from the hydrogen atom, parts which are called "electrons" and which have a mass of approximately one two-thousandth that of the hydrogen atom. These electrons may be obtained from matter by other processes also, such as X-rays, ultra-violet light, and high temperatures. One investigator has estimated that the size of an electron bears to the size of an atom about the same relation as the size of a pinhead bears to the size of the dome on St. Paul's Cathedral.
In all, there are about ninety kinds of atoms: these are the "elements" of the chemist. Professor Millikan of Chicago University has found the mass, in grams, of a hydrogen atom to be 1.62x10-24.
Experiments indicate that it would require about 100,000,000 average molecules laid side by side in a straight line to cover a distance of one centimeter.
See Sir Joseph Thomsen's Corpuscular Theory of Matter (Scribners), Soddy's Interpretation of Radium, Cameron's Radium and Radioactivity (Romance of Science Series), Kimball's Properties of Gases (Houghton Mifflin Company). Henry Crew.
A'treus, according to the Greek legend, the son of Pelops and Hippodamia. By some versions he is accounted the father of Agamemnon and Menelaus, while others say he was their grandfather but that he reared them as sons. The whole story of the house of Atreus is one of bloodshed, the series of crimes beginning with the murder of Chrysippus by his half brothers, Atreus and Thyestes, and ending with the murder of Clytemnestra and her husband by Clytemnestra's son, Orestes. Because of the first murder, Pelops pronounced the curse upon his sons that they and their posterity should perish by means of one another.
At'tar of Roses (from the Arab word for perfume), the oil extracted from the petals of the rose. It is prepared from rose-water in Persia, India and other eastern countries by setting it out during the night in large open vessels, and early in the morning skimming off the oil which floats at the top. It is very costly, and is often adulterated with sandalwood and other oils. Half an ounce of the oil can be made from 200,000 well-grown roses, and this amount, when manufactured, is worth about $40. The oil is at first colorless, but later shows a yellow tint. The oil of Adrianople and of Ghazipoor in Hindustan is considered the best. The region about Ghazipoor is one great expanse of roses.
At'tica, one of the divisions of ancient Greece, with Athens as its capital. Its area was about 640 square miles. It is of the shape of a triangle, having its northeast and southwest sides washed by the sea, and joined to the mainland on the north. As early as the time of Solon it was well cultivated, and produced wine and corn. Figs, olives and grapes are still grown, and goats and sheep are raised. Today Attica and Bceotia together form a division or government in the kingdom of Greece, with an area of 2,472 square miles and a population of 341,247.
At'tila, the Scourge of God, -'born about 406 A. D., was son of the king of the Huns, and in 434 succeeded his uncle as king of countless hordes scattered over the north of Asia and Europe. He was regarded by^ the Huns with reverence but by the Christians with dread. He is said to have received the title Scourge of God from a hermit in Gaul. His power at one time reached from the Rhine to the frontiers of China. In 447 he laid waste the entire region between the Black Sea and the Mediterranean. All the people, it is related, were either destroyed or forced to follow
