578 
ARTICLE XXV. 
Note on a Capillary Phenomenon observed by Dr. Youne. 
By Prof. Mossorri*. 
1. L HAVE had the honour of presenting within the last few 
days, to the different members of this Section, a copy of a lecturet 
lately published in the 98th volume of the Biblioteca Italiana, on 
the Theory of Capillary Attraction. My object in so doing was 
to call your attention to the simple notions from which the said 
theory has been deduced, to apply it now to the explanation of 
a phenomenon which has been investigated, I think without 
success, by the illustrious Poisson in his excellent work Nouvelle | 
Théorie de ? Action Capillaire, p. 141. 
The phenomenon to which I allude relates to the equilibrium 
of two fluids placed one upon the other in a capillary tube, and 
is the memorable one that Dr. Young brought forward against 
the theory of Laplace. Dr. Young, as is well known, observed 
that if a drop of oil be poured into a tube immersed in water, 
and in which the fluid has risen to its due height, the level of 
the external surface of the oil descends perceptibly below the 
height at which the external surface of the column of water pre- 
viously stood. Laplace’s formule, as well as Poisson’s, which 
are identical, do not agree with this descent, and only the latter 
of these authors contented himself with observing that the upper 
surface of the small fluid column will exhibit in the direction of 
its axis a slight depression of level, owing to the greater con- 
cavity that the said surface acquires. So slight a depression 
cannot, however, be the same as that which attended the fact 
mentioned by Dr. Young, since this philosopher, speaking of 
seeing the small fluid column descend, uses the word conspi- 
cuously. 
2. The chief cause that gives rise to capillary phenomena is 
that tension or contractile force that fluids acquire along their 
surface, which I have shown to depend on the rapid rarefaction 
to which liquids are subject near their surface. When the sur- 
face of the fluid is free this contractile force is greatest, since 
* Read in part at the Section of Nat. Phil., Chemistry and Mathematics of 
the Second Meeting of the Italian Scientific Association at Turin, Sept. 1840. 
+ Vide Art. XXIII. and note, p. 558. 
