OBSERVED BY DR. YOUNG. 585 
from the fact, that he considers the whole weight of the small 
column composed of the two fluids to be equivalent to the action 
that would be exerted by the tension of the surface of the lower 
fluid, combined with its curvature, as though the tube did not 
contain any other fluid above, whereas, in my opinion, the said 
_ weight is sustained by the united actions of the tension, both 
_ of the superior surface of the upper fluid and of the lower fluid, 
combined with their respective curvatures, but estimating the 
“tension along the surface of the latter fluid according to the 
nature of the fluid that is placed upon it. Poisson has attempted 
Bemother demonstration of his proposition in Art. 72; but we can 
i easily perceive that his demonstration is defective, since in esti- 
- mating the second part of the force which he denotes by R, he 
has not taken into account the state of tension of the upper 
fluid; for if the portion of the small fluid column that stands 
_ above the natural level were not slightly though imperceptibly 
" rarefied, and did not acquire a state of tension, the upper fluid 
3 parts would be unable to sustain the lower parts, and the column 
would break and fall when placed in vacuo*. 
For these reasons we believe that the assertions of Gay-Lussac, 
_ quoted by Poisson, Art. 74, ought not to be so placed, but rather 
taken with the formula by means of which we calculated Cava- 
_ lier Avogadro’s experiment, viz. formula (d.), and from this we 
_ Shall obtain the values of the tension of the surface of the mer- 
_ eury when perceptibly in contact with water and with alcohol, 
expressed respectively by 
q T,= 27794, T, = 2°63 9A, 
_ A being the density of the mercury. 
_ 8. Ihave thought fit to point out these examples, to show 
how much greater facility the new point of view under whic 
the theory of capillary action has been lately expounded affords 
in its applications where no misunderstanding occurs. On the 
other hand, these results by no means invalidate the principles 
laid down in the Nowvelle Théorie de l Action Capillaire. It is to 
the celebrated author of that treatise that we are indebted for 
the calculation of the pressure or tension in bodies formed of 
discontinuous molecules, for the discovery of the necessity of a 
rapid rarefaction towards the surfaces of fluids in order that 
rr 
« In the atmosphere the pressure of the air, which balances itself, condenses 
the fluid mass and produces in it a uniform increase of repulsion between the 
patts, which does not alter the conditions of equilibrium that obtain in vacuo. 
