substances which furnish them suitable food. These are known as culture media. Among the more common of these are bouillon, milk, and potato. In order to make a medium which is solid at ordinary temperatures, but which may be liquefied by slight heating, gelatin or agar is often added to the bouillon. Thus, if a drop of fluid containing bacteria which we wish to study be added to warm melted gelatin or agar bouillon, the latter may be poured out in a thin layer on a cold plate, where it immediately hardens, thus fixing the bacteria and preventing their moving about in the medium. If the dilu- tion be sufficient to separate the bacteria well, as growth occurs, minute specks are seen dotting the surface of the plate, and each one of these specks marks the reproduction area of a single bacterium. It is called a colony, and each such colony consists of a single species of bacteria. By taking a bit from one of these spots or colonies and transferring it to some other sterile medium, there results a growth which is free from all life other than the single species of bacteria. This is known as a 'pure culture.' Many different species of bacteria so closely resemble one another in appearance that they cannot be differentiated by the micro- scope. Each species has, however, certain peculiarities of growth and development. Thus recourse is often had to cultivation upon dif- ferent media, where their life-histories may be studied and the identity of a particular species determined. For the very careful study of bac- teria, many different modifications of the culture- media have been devised. The medium is some- times colored by litmus, to show any reaction the growth of the bacteria may produce. Sugar is sometimes added to the medium, which is then placed in a crooked tube closed at one end, so that if the species causes fermentation it may be noted by the collecting of gas at the closed end of the tube. All media are sterilized before using, to destroy any bacteria that may be present. After inoculation the media are kept at room temperature, or more often are placed in an incubator at about blood-heat.
For microscopical study, bacteria may be simply placed under a thin glass cover upon a glass slide. They may be studied unstained, but are more often stained in various ways, to render them more distinct or to bring out certain peculiarities. Some of the bacteria, e.g. the tubercle bacillus (see Tuberculosis), depend for their recognition very largely upon their specific reaction to certain stains, and can be differentiated only in this way. They are usually studied with an oil-immersion lens of not less than 1000 diameter magnification. For history and bibliography, see Disease, Germ Theory of.
BACTE'RIAL FIL'TER-BEDS. See Sewage Disposal.
BACTRIA. The name given in ancient times to a province or country in Central Asia extending northward from the Hindu-Kush Mountains as far as the river Oxus (Amu or Jihon), and called also Bactriana by classical writers. The exact extent of this land in early times cannot now be determined, but it must have been considerable; and, although barren to the north, its southern regions were rich in pasture lands, which made Bactria famous in antiquity, among other things, for its fine breed of horses. In history it has been the seat of a number of powerful rulers, who are best known as the successors of Alexander the Great in the East. What was formerly Bactria is now included in parts of Afghanistan and Asiatic Russia.
The name ot the province or country appears in the Old Persian Inscriptions (Bh. i. IG; Dar. Pers. e. 10; NR. a. 23) as Buxtri, i.e. Bakhtri. It is written in the Avesta BuxSi. From this latter came the intermediate form Baxli, Sanskrit Balilika, Balhika "Bactrian," Armen. Bahl, and by transposition, the Mod. Pers. Balx, i.e. Balkh. The capital city, Balkh (q.v. ), or earlier Bactra, was situated on the Dargidus or Bactrus (now Dehas). This city seems also to be known in the classics as Zari- aspa. In the Avesta (Vendidad 1.6), and else- where, it receives the title 'beautiful' (Av. srira) , to which the attri!)ute 'with banners on high' is appended. This additional designation of the capital of Bactria in ancient times was ap- parently due to the custom, mentioned by Masudi and Yakut, of pious pilgrims hanging green silk Iranners from the walls of the Temple Naubehar. This latter name has been supposed to refer to a Buddhist 'New Cloister' (nava-vihara) , as we know that Balkh, in the Province of Bactria, was a flourishing seat of Buddhism in the early Christian centuries.
Bactria is supposed to have been originally the centre of a powerful kingdom founded more than a thousand years before the Christian era, and exercising extensive sway throughout Iran. The strongest support for the existence of such a kingdom, antedating the Median rule and the Achæmenian sovereignty of the Persian Empire, will be found in Duncker, Geschichte den Altertums (Leipzig, 1878-86). One of the principal arguments brought forward in favor of such a view is the story of the campaign of Ninus of Assyria against a Bactrian king, Zoroaster, as mentioned by Ctesias, and recounted with some variations by Trogus Pompeius (in Justin), Eusebius, and others. The record seems to contain some allusion to Zoroaster, the Prophet of ancient Iran (q.v.), whose patron was Vishtaspa, and whose death occurred at Balkh, according to Firdausi and others, when the city was stormed by the Turanians. For this and similar reasons Bactria is often regarded as the cradle of the ancient Magian religion. (See Zoroaster.) The bearing of all these passages in their historic light is discussed in Jackson, Zoroaster, the Prophet of Ancient Iran (New York, 1899). But considerable doubt is attached to the antiquity of the story of Zoroaster's death at Balkh; and so high an authority on Persian history as Justi disclaims the existence of a Bactrian kingdom before the time of Alexander the Great. Yet others may hold a different opinion. However the case may be, there is no doubt that Cyrus subjugated Bactria and made it a Persian province; and Darius in his inscriptions includes Bactria as one of the countries tributary to the Achæmenian power. Herodotus enumerates the forces which Bactria contributed to the Persian invasion of Greece.
From the time of Alexander, the history of Bactria becomes clearer, and may be followed with more detail. When he conquered the rest of Iran, Bactria likewise fell before his power, and he made Roxana, daughter of the Bactrian ruler Oxyartes, his wife, B.C. 327. On leaving Persia,
