se of Lectures on Natural Philosophy and the
Mechanical Arts," published in 1807, in which he propounded the
undulatory theory of light, and the principle of the interference of
rays; the hieroglyphic inscriptions of Egypt occupied much of his
attention, and he is credited with having anticipated Champollion in
discovering the key to them (1773-1829).

YOUNG MEN'S CHRISTIAN ASSOCIATION, an association founded in London
in 1844, for the benefit of young men connected with various dry-goods
houses in the city, and which extended itself over the other particularly
large cities throughout the country, so that now it is located in 1249
centres, and numbers in London alone some 14,000 members; its object is
the welfare of young men at once spiritually, morally, socially, and
physically.

YOUNG PEOPLE'S SOCIETY OF CHRISTIAN ENDEAVOUR, a society established
in 1881 by Dr. F. E. Clark, Portland, Maine, U.S., in 1898; has a
membership of three and a quarter million; it is undenominational, but
evangelical apparently, and its professed object is "to promote an
earnest Christian life among its members, to increase their mutual
acquaintanceship, and to make them more useful in the service of God."

YOUNGSTOWN (45), a town in Ohio, U.S., with large iron factories;
is in the heart of a district rich in iron and coal.

YPRES (16), an old Belgian town in West Flanders, 30 m. SW. of
Bruges; was at one time a great weaving centre, and famous for its diaper
linen; has much fallen off, though it retains a town-hall and a
cathedral, both of Gothic architecture in evidence of what it once was;
it was strongly fortified once, and has been subjected to many sieges;
the manufacture of thread and lace is now the most important industry.

YRIARTE, CHARLES, French litterateur, born in Paris, of Spanish
ancestry; has written works dealing with Spain, Paris, the Franco-German
War, Venice, &c.; _b_. 1832.

YRIARTE, THOMAS DE, Spanish poet; studied at Madrid; was editor of
the _Madrid Mercury_; his principal works "Musica," a poem, and "Literary
Fables" (1750-1790).

YSTAD, a seaport in the extreme S. of Sweden, with a commodious
harbour, and a trade chiefly in corn.

YSTRADYFODWG (88), a township in Glamorgan, in a rich mining
district.

YTTRIUM, a rare metal always found in combination with others, and
is a blackish-gray powder; the oxide of it, yttria, is a soft whitish
powder, and when ignited glows with a pure white li