French jurist and early advocate of the cottage reformatory for juvenile offenders, which anticipated the English system of Borstal reformatories established in the 20th century.

During his time as a judge (1821–40), Demetz was concerned with the problem of sentencing juveniles to live among hardened criminals. He therefore founded in 1840 the farm colony of Mettray in the Loire valley, near Tours. Small groups of juvenile delinquents were assigned to separate cottages, which were equipped with workshops and were supervised by a family head and two assistants. Outdoor work and recreation, as well as religious and primary education, were provided. The arrangement at Mettray resembled that of the later Borstal institutions in England. Demetz wrote several books in which he set forth his theories on penology.