96 The Rev. J. Challis on the Molecular Forces of Fluids. 
pends on the law, intensity, and sphere of activity of its mo- 
lecular forces. Admitting the superficial variation of density 
to be negligible, and the fluids to be incompressible, H_ will 
be a measure of their cohesiveness, or the inverse of it a mea- 
sure of their fluidity, as it is proportional to the effect of their 
molecular attractions acting under the same circumstances. 
(Laplace’s Treatise, art. 12. and Supplement, p. 18.) On 
this account it formerly appeared to me simplest to suppose 
this quantity to vary ase; and the first experiments of 
M. Link favoured this idea by assigning to different fluids 
the same height of ascent. Those subsequently made, which 
are the more accurate, do not give the same result, but suffi- 
ciently prove that the heights are not as the specific gravities, 
and consequently that H does not vary as 9°, as is usually 
supposed. Probably nothing can be determined respecting 
it d priori. The last-mentioned experiments gave the fol- 
lowing results when the fluids ascended between glass plates 
thoroughly moistened:—height x specific gravity = 5:3, for 
sulphuric ether; 6°7, for alcohol; 10°7, for liquid caustic 
alkali; 10°9, for liquid ascetic [carbonated ?] alkali; 12:5, for 
water; 15°6, for muriatic acid; 16°8, for nitric acid; 20°3, 
for sulphuric acid. According to what is said above, these 
numerical quantities are in the order of cohesiveness, sul- 
phuric ether being the least cohesive body, or possessing the 
greatest degree of fluidity. 
The phenomenon of endosmose may be appealed to as in- 
dicating a great attractive energy in partially fluid substances. 
When water is on one side of the porous membrane, and an 
imperfect fluid, as treacle, or solution of gum, on the other, 
the latter is found to draw the water powerfully through the 
pores. The force exerted at any time appears by experiment 
to be in this case proportional to the difference of the den- 
sities of the fluids on the opposite sides of the membrane. 
The subjects treated of in this paper are of such a nature 
as scarcely to admit of any very definite discussion. Since, 
however, the degree in which capillary phenomena may be 
affected by a variation of density at the surfaces of fluids is at 
present quite unknown, it seemed desirable at least to ascer- 
tain whether an approximation to the truth is obtained when 
that variation is neglected; and, perhaps, the preceding rea- 
sons, connected with the nature of fluidity, may make it pro- 
bable that such is the case. 
Papworth St. Everard, Dec. 10, 1835, 
