specific cohesion double that of mercury. Zinc, iron and
palladium, three times that of mercury, and sodium, six times that of
mercury.

RELATION OF SURFACE-TENSION TO TEMPERATURE

It appears from the experiments of Brunner and of Wolf on the ascent of
water in tubes that at the temperature t deg. centigrade

T = 75.20 (1 - 0.00187t) (Brunner);
= 76.08 (1 - 0.002t + 0.00000415t^2), for a tube .02346 cm. diameter
(Wolf);
= 77.34 (1 - 0.00181t), for a tube .03098 cm. diameter (Wolf).

Lord Kelvin has applied the principles of Thermodynamics to determine
the thermal effects of increasing or diminishing the area of the free
surface of a liquid, and has shown that in order to keep the temperature
constant while the area of the surface increases by unity, an amount of
heat must be supplied to the liquid which is dynamically equivalent to
the product of the absolute temperature into the decrement of the
surface-tension per degree of temperature. We may call this the _latent
heat of surface-extension_.

It appears from the experiments of C. Brunner and C.J.E. Wolf that at
ordinary temperatures the latent heat of extension of the surface of
water is dynamically equivalent to about half the mechanical work done
in producing the surface-extension.

REFERENCES.--Further information on some of the matters discussed
above will be found in Lord Rayleigh's _Collected Scientific Papers_
(1901). In its full extension the subject of capillarity is very wide.
Reference may be made to A.W. Reinold and Sir A.W. Rucker (_Phil.
Trans._ 1886, p. 627); Sir W. Ramsay and J. Shields (_Zeitschr.
physik. Chem._ 1893, 12, p. 433); and on the theoretical side, see
papers by Josiah Willard Gibbs; R. Eotvos (_Wied. Ann._, 1886, 27, p.
452); J.D. Van der Waals, G. Bakker and other writers of the Dutch
school. (J. C. M.; R.)

FOOTNOTES:

[1] In this revision of James Clerk Maxwell's classical article in
the ninth edition of the _Encyclopaedia Britannica_, additions are
marked by square brackets.

[2] See Enrico Betti, _Teoria della Capillarita: Nuovo Cimento_
(1867); a memoir by M. Stahl, "Ueber einige Punckte in der Theorie
der Capillarerscheinungen," _Pogg. Ann._ cxxxix. p. 239 (1870); and
J.D. Van der Waal's _Over de Continuiteit van den Gasen
Vloeistoftoestand_. A good account of the subject from a mathematical
point of view will be found in James Challis's "Report