BIBLIOGRAPHY.-M. W. Travers, Experimental Study of Gases (1902) ; Claude, Liquid Air, Oxygen and Nitrogen (trans. by Cottrell, 191o) ; H. C. Greenwood, Industrial Gases (1920) ; Sir J. A. Ewing, The Mechanical Production of Cold (1921) ; "Generation and Utiliza tion of Cold," Transactions of the Faraday Society (1922) . See also Industrial Gases (quarterly journal) ; The Dictionary of Applied Physics; Publications of the International Congresses on Refrigeration; Publications from the Cryogenic Laboratory at Leyden. (M. W. T.) LIQUEURS, the general term applied to perfumed or fla voured potable spirits, sweetened by the addition of sugar. The term "liqueur" is also used for certain wines and unsweetened spirits of very superior quality, or remarkable for their bouquet, such as tokay or fine old brandy or whisky.
The basis of all the "liqueurs," proper consists of (a) relatively strong alcohol or spirit, which must be as pure and neutral as possible ; (b) sugar or syrup; and (c) flavouring matters. There are three distinct main methods of manufacturing liqueurs. The first, by which liqueurs of the highest class are prepared, is the "distillation" or "alcoholate" process. This consists in macerat ing various aromatic substances such as seeds, leaves, roots and barks of plants, etc., with strong spirit and subsequently dis tilling the infusion so obtained, generally in the presence of a whole or a part of the solid matter. The mixture of spirit, water and flavouring matters which distils over is termed the "alcohol ate." To this is added a solution of sugar or syrup, and frequently colouring matter in the shape of harmless vegetable extracts or burnt sugar, and a further quantity of flavouring matter in the shape of essential oils or clear spirituous vegetable extracts. The second method of making liqueurs is that known as the "essence" process. It is employed, as a rule, for cheap and inferior articles; the process resolving itself into the addition of various essential oils, either natural or artificially prepared, and of spirituous ex tracts to strong spirit, filtering and adding the saccharine matter to the clear filtrate. The third method of manufacturing liqueurs
is the "infusion" process, in which alcohol and sugar are added to various fresh fruit juices. Liqueurs prepared by this method are frequently called "cordials." The French, who excel in the preparation of liqueurs, grade their products, according to their sweetness and alcoholic strength, into cremes, huiles or baumes, which have a thick, oily consistency; and eaux, extracts or elixirs, which, being less sweetened, are rel atively limpid. Liqueurs are also classed, according to their commercial quality and composition, as ordinaires, demifines, fines and sur-fines. Certain liqueurs, containing only a single flavouring ingredient, or having a prevailing flavour of a particu lar substance, are named after that body, for instance, creme de vanille, anisette, kiimmel, creme de menthe, etc. On the other hand, many well-known liqueurs are compounded of very num erous aromatic principles. The nature and quantities of the flavouring agents employed in the preparation of liqueurs of this kind are kept strictly secret, but numerous "recipes" are given in works dealing with this subject. Among the substances fre quently used as flavouring agents are aniseed, coriander, fennel, wormwood, gentian, sassafras, amber, hyssop, mint, thyme, angelica, citron, lemon and orange peel, peppermint, cinnamon, cloves, iris, caraway, tea, coffee and so on.
The alcoholic strength of liqueurs ranges from close on 8o% of alcohol by volume in some kinds of absinthe, to 27% in anisette. The liqueur industry is a very considerable one in France. Among other well-known trade liqueurs may be mentioned maraschino, which takes its name from a variety of cherry—the marasca grown in Dalmatia, the centre of the trade being at Zara; kiimmel, the flavour of which is largely due to caraway seeds; allasch, which is a rich variety of kilmmel; and cherry and other "fruit" brandies and whiskies, the latter being perhaps more properly termed cordials. (See also BENEDICTINE; CHARTREUSE ; CREME DE MENTHE; KUMMEL.) See Duplais, La Fabrication des liqueurs; and Rocques, Les Eaux de-vie et liqueurs.