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 fish itself; this acts as an ovipositor by means of which the comparatively few and large eggs (3 millimetres in diameter) are introduced through the gaping valves between the branchiae of pond mussels (Unio and Anodonta), where, after being inseminated, they undergo their development, the fry leaving their host about a month later. The mollusc reciprocates by throwing off its embryos on the parent fish, in the skin of which they remain encysted for some time, the period of reproduction of the fish and the mussel coinciding.

 BITTERN, a genus of wading birds, belonging to the family Ardeidae, comprising several species closely allied to the herons, from which they differ chiefly in their shorter neck, the back of which is covered with down, and the front with long feathers, which can be raised at pleasure. They are solitary birds, frequenting countries possessing extensive swamps and marshy grounds, remaining at rest by day, concealed among the reeds and bushes of their haunts, and seeking their food, which consists of fish, reptiles, insects and small quadrupeds, in the twilight. The common bittern (Botaurus stellaris) is nearly as large as the heron, and is widely distributed over the eastern hemisphere. Formerly it was common in Britain, but extensive drainage and persecution have greatly diminished its numbers and it is now only an uncertain visitor. Not a winter passes without its appearing in some numbers, when its uncommon aspect, its large size, and beautifully pencilled plumage cause it to be regarded as a great prize by the lucky gun-bearer to whom it falls a victim. Its value as a delicacy for the table, once so highly esteemed, has long vanished. The old fable of this bird inserting its beak into a reed or plunging it into the ground, and so causing the booming sound with which its name will always be associated, is also exploded, and nowadays indeed so few people in Britain have ever heard its loud and awful voice, which seems to be uttered only in the breeding-season, and is therefore unknown in a country where it no longer breeds, that incredulity as to its booming at all has in some quarters succeeded the old belief in this as in other reputed peculiarities of the species. The bittern in the days of falconry was strictly preserved, and afforded excellent sport. It sits crouching on the ground during the day, with its bill pointing in the air, a position from which it is not easily roused, and even when it takes wing, its flight is neither swift nor long sustained. When wounded it requires to be approached with caution, as it will then attack either man or dog with its long sharp bill and its acute claws. It builds a rude nest among the reeds and flags, out of materials which surround it, and the female lays four or five eggs of a brownish olive. During the breeding season it utters a booming noise, from which it probably derives its generic name, Botaurus, and which has made it in many places an object of superstitious dread. Its plumage for the most part is of a pale buff colour, rayed and speckled with black and reddish brown. The American bittern (Botaurus lentiginosus) is somewhat smaller than the European species, and is found throughout the central and southern portions of North America. It also occurs in Britain as an occasional straggler. It is distinguishable by its uniform greyish-brown primaries, which want the tawny bars that characterize B. stellaris. Both species are good eating.

Bittern.

 BITTERN (from “bitter”), the mother liquor obtained from sea-water or brines after the separation of the sodium chloride (common salt) by crystallization. It contains various magnesium salts (sulphate, chloride, bromide and iodide) and is employed commercially for the manufacture of Epsom salts (magnesium sulphate) and bromine. The same term is applied to a mixture of quassia, iron sulphate, cocculus indicus, liquorice, &c., used in adulterating beer.

 BITTERS, the name given to aromatized (generally alcoholic) beverages containing a bitter substance or substances, used as tonics, appetizers or digestives. The bitterness is imparted by such substances as bitter orange rind, gentian, rhubarb, quassia, cascarilla, angostura, quinine and cinchona. Juniper, cinnamon, carraway, camomile, cloves and other flavouring agents are also employed in conjunction with the bitter principles, alcohol and sugar. Some bitters are prepared by simple maceration and subsequent filtration (see ), others by the more complicated distillation process. Those prepared by the latter process are the finer commercial articles. Bitters are usually sold under the name of the substance which has been used to give them the predominant flavour, such as orange, angostura or peach bitters, &c. The alcoholic strength of bitters varies, but is generally in the neighbourhood of 40% of alcohol. Some bitters, although possessing tonic properties, may be regarded as beverages pure and simple, notwithstanding the fact that they are seldom consumed in an undiluted state; others again, are obviously medicinal preparations and should be treated as such.

 BITUMEN, the name applied by the Romans to the various descriptions of natural hydrocarbons, the word petroleum not being used in classical Latin. In its widest sense it embraces the whole range of these substances, including natural gas, the more or less liquid descriptions of petroleum, and the solid forms of asphalt, albertite, gilsonite or uintahite, elaterite, ozokerite and hatchettite. To distinguish bitumen intermediate in consistency between asphalt and the more liquid kinds of crude petroleum, the term maltha (Latin) is frequently employed. The bitumens of chief commercial importance may be grouped under the three headings of (1) natural gas, (2) petroleum, and (3) asphalt, and will be found fully described under these titles. In the scriptures there are numerous references to bitumen, among which the following may be quoted:—In Genesis ix. 3, we are told that in the building of the tower of Babel “slime had they for mortar,” and in Genesis xiv. 10, that the vale of Siddim “was full of slime-pits,” the word slime in the latter quotation from our version appearing as bitumen in the Vulgate. Herodotus alludes to the use of the bitumen brought down by the Is, a tributary of the Euphrates, as mortar in building the walls of Babylon. Diodorus, Curtius, Josephus, Bochart and others make similar mention of this use of bitumen, and Vitruvius tells us that it was employed in admixture with clay.

In its various forms, bitumen is one of the most widely distributed of substances. It occurs, though sometimes only in small quantity, in almost every part of the globe, and throughout the whole range of geological strata, from the Laurentian rocks to the most recent members of the Quaternary period. Although the gaseous and liquid forms of bitumen may be regarded as having been formed in the strata in which they are found or as having been received into such strata shortly after formation, the semi-solid and solid varieties may be considered to have been produced by the oxidation and evaporation of