which the natural warm spring was once conducted. Behind these is a fine triumphal arch, whence runs a colonnade. Ruins of several churches survive, and also of a large basilica. There is a sulphureous pool which may represent the "Plutonium," but it has no such deadly power as was ascribed to that pond. Ramsay thinks that the "Plutonium" was obliterated by Christians in the 4th century. Over 300 inscriptions have been collected, mostly sepulchral, whence Ramsay has deduced interesting facts about the very early Christian community which existed here. The site has been often visited and described, and was systematically examined in 1887 by parties under W. M. Ramsay and K. Humann respectively. See K. Humann, _Altertumer v. Hierapolis_ (1888); Sir W. M. Ramsay, _Cities and Bishoprics of Phrygia_, vol. i. (1895). (C. W. W.; D. G. H.) HIERARCHY (Gr. [Greek: hieros], holy, and [Greek: archein], to rule), the office of a steward or guardian of holy things, not a "ruler of priests" or "priestly ruler" (see Boeckh, _Corp. inscr. Gr._ No. 1570), a term commonly used in ecclesiastical language to denote the aggregate of those persons who exercise authority within the Christian Church, the patriarchate, episcopate or entire three-fold order of the clergy. The word [Greek: hierarchia], which does not occur in any classical Greek writer, owes its present extensive currency to the celebrated writings of Dionysius Areopagiticus. Of these the most important are the two which treat of the celestial and of the ecclesiastical hierarchy respectively. Defining hierarchy as the "function which comprises all sacred things," or, more fully, as "a sacred order and science and activity, assimilated as far as possible to the godlike, and elevated to the imitation of God proportionately to the Divine illuminations conceded to it," the author proceeds to enumerate the nine orders of the heavenly host, which are subdivided again into hierarchies or triads, in descending order, thus: Seraphim, Cherubim, Thrones; Dominations, Virtues, Powers; Principalities, Archangels, Angels. These all exist for the common object of raising men through ascending stages of purification and illumination to perfection. The ecclesiastical or earthly hierarchy is the counterpart of the other. In it the first or highest triad is formed by baptism, communion and chrism. The second triad consists of the three orders of the ministry, bishop or hierarch, priest and minister or deacon ([Greek: hierarches, hiereus, leitourgos]); this is the earliest known instance in which the title hierarch is applied to a bishop. The third or lowest triad is made up of monks, "initiated" and catechumens. To Dionysius may be traced, through Thomas Aquinas and other Catholic writers of the intervening period, the definition of the term usually given by Roman Catholic writers--"coetus seu ordo praesidum et sacrorum ministrorum ad regendam ecclesiam