olecular velocity in the cases examined of
non-electrolytes dissolved in water at 18 deg. at 2.5 grams per litre.

BIBLIOGRAPHY.--The best introduction to the study of theories of
diffusion is afforded by O. E. Meyer's Kinetic _Theory of Gases_,
translated by Robert E. Baynes (London, 1899). The mathematical
portion, though sufficient for ordinary purposes, is mostly of the
simplest possible character. Another useful treatise is R. Ruhlmann's
_Handbuch der mechanischen Warmetheorie_ (Brunswick, 1885). For a
shorter sketch the reader may refer to J. C. Maxwell's _Theory of
Heat_, chaps, xix. and xxii., or numerous other treatises on physics.
The theory of the semi-permeable membrane is discussed by M. Planck
in his _Treatise on Thermodynamics_, English translation by A. Ogg
(1903), also in treatises on thermodynamics by W. Voigt and other
writers. For a more detailed study of diffusion in general the
following papers may be consulted:--L. Boltzmann, "Zur Integration der
Diffusionsgleichung," _Sitzung. der k. bayer. Akad math.-phys. Klasse_
(May 1894); T. des Coudres, "Diffusionsvorgange in einem Zylinder,"
_Wied. Ann._ lv. (1895), p. 213; J. Loschmidt,
"Experimentaluntersuchungen uber Diffusion," _Wien. Sitz._ lxi., lxii.
(1870); J. Stefan, "Gleichgewicht und ... Diffusion von Gasmengen,"
_Wien. Sitz._ lxiii., "Dynamische Theorie der Diffusion," _Wien.
Sitz._ lxv. (April 1872); M. Toepler, "Gas-diffusion," _Wied. Ann._
lviii. (1896), p. 599; A. Wretschko, "Experimentaluntersuchungen uber
die Diffusion von Gasmengen," _Wien. Sitz._ lxii. The mathematical
theory of diffusion, according to the kinetic theory of gases, has
been treated by a number of different methods, and for the study of
these the reader may consult L. Boltzmann, _Vorlesungen uber
Gastheorie_ (Leipzig, 1896-1898); S. H. Burbury, _Kinetic Theory of
Gases_ (Cambridge, 1899), and papers by L. Boltzmann in _Wien. Sitz._
lxxxvi. (1882), lxxxvii. (1883); P. G. Tait, "Foundations of the
Kinetic Theory of Gases," _Trans. R.S.E._ xxxiii., xxxv., xxvi., or
_Scientific Papers_, ii. (Cambridge, 1900). For recent work reference
should be made to the current issues of _Science Abstracts_ (London),
and entries under the heading "Diffusion" will be found in the general
index at the end of each volume. (G. H. BR.)

DIGBY, SIR EVERARD (1578-1606), English conspirator, son of Everard
Digby of Stoke