ATRATO ATREUS 89 There are seeming exceptions to this law. Two atoms of a monad element, a%j>otassium, may unite with one, two, three, four, or five atoms of a polyad element, as sulphur. By an examination of the graphic formulas of these compounds, K S K, K S S K, K S S S K, &c., it is seen that any number of atoms of a polyad element may unite with two atoms of a monad, provided they be interposed between the latter. When thus placed, they are said to perform a linking function in the compound. The atomicity of an element is its highest equivalence, and the compound form is then said to be normal or saturated. Yet the equivalence of atoms is not always the same ; an atom may form sev- eral compounds of the same substance. Ele- ments of even equivalence, in which the atomic poles are in pairs, are called artiads; those of uneven equivalence, in which the poles are odd, are termed perissads. Prof. Barker states that the equivalence " always increases or di- minishes by two ; so that an atom of the same element may in different compounds have an equivalence of 1, 3, 5, or 7, or of 2, 4, or 6. A perissad atom can never become an artiad atom by such a change, nor can an artiad become a perissad." This variation of atomic equiva- lence is accounted for on the hypothesis that the bonds of an atom are capable of saturating each other in pairs. A pentad may thus be- come a triad and a monad successively, and a hexad may be converted into a tetrad or into
- i dyad, as follows
Pentad. Triad. Monad. Hexad. Tetrad. Dyad. It follows from this view that only the atoms of those free elements can be considered as existing separately in which the number of bonds is even. The others can only exist in combination with each other, forming poly- atomic molecules. Free hydrogen cannot be H, because its bond is unsatisfied ; it must therefore be H H, that is, united with itself, forming what we might call hydride of hydro- gen. Chlorine is not Cl , but 01 Cl, and free oxygen is not O , but O=O. Com- pounds are formed by replacement, and chem- ical science thus becomes rooted in atomic ca- pacity. While therefore in the last quarter of a century chemical philosophy has undergone a total revolution, the atomic theory has not only been maintained and strengthened, but it is doubtful if the advance could have been made without its assistance. ATRATO, a river of Colombia, South America, rises near lat. 5 20' N. and Ion. 76 50' W., and flows nearly due N. for about 250 m. to the gulf of Darien. The bar at its mouth being crossed, it has a wide channel not less than 35 ft. deep for the first 96 in., with a fall not ex- crcding 2J inches to the mile; and for 4-2 m. further a channel exceeding 18 ft. in depth can he cleared ; while the distance across to the Pacific ocean, from which the river is separated by one of the lowest ranges of the Andes, does not exceed 50 m., and western branches of the Atrato are said to almost meet rivers from the Pacific having their source in this dividing ridge. Examinations have been made with the view of determining the practicability of con- structing a ship canal by this river, to connect the Caribbean sea with the Pacific. The latest was by the United States government in 1871. The route which promised the least difficulty between the middle branch of the Atrato and the Jurador, emptying into the Pacific, would require 48 m. of canal ; the height of the water- shed, which must be excavated or tunnelled, being more than 500 ft. The Atrato for nearly its whole length runs through a low swampy region, which is entirely overflowed by fresh- ets. Quibdo, on its upper course, is the only town of any consequence on the river. It is a miserable place of 1,500 inhabitants, mostly blacks, with some Indians and a few whites. It is situated on several isolated hillocks of gravel and clay, in the midst of the swampy region which extends all around. The temperature of the region is close and sultry, and the rainy season continues all the year. Gold is found in tine dust in the bed and banks of the Atrato, at and above Quibdo, and also of the different branches of the river. Some portions of the country are described as highly auriferous. Above Quibdo the Atrato receives several branches, of which the Quito is the most im- portant. Were it not for the incessant fluctua- tions of this stream, which within a few hours frequently reduce it from its ordinary ample channel depth of 7 ft. or more to 5 or 6 It. or even less, the Quito would present with the Atrato an uninterrupted steamboat thorough- fare of no less than 252 m. from the gulf of Darien. The Quito is wholly in the gold re- gion, and its branches appear to lie in the rich- est portion of it. The caoutchouc tree abounds. ATREBATES, or Atrebatii, a people of Belgic Gaul, whose name appears in the modern Artois. They joined a confederation against Caesar, and furnished a contingent of 15,000 troops. A colony of them settled in Britain, in the modern Berkshire and Wiltshire. TKKI S, a legendary hero of Greece, son of Pelops and Hippodamia. On the death of his son Plisthenes, Atreus married his widow Aerope, who was or became the mother of Agamemnon and Menelaus, commonly known as the Atridse. She was seduced by Thy- estes, the brother of Atreus, and the latter slew the twin offspring of this adultery and served them at a banquet to the seducer. Atreus afterward married his brother's daugh- ter Pelopia, who was already pregnant with /Egisthus by her own father. The child was I exposed, but miraculously preserved, and the mother committed suicide. The crimes and misfortunes of the family, springing from the
