370 
ON THE MECHANICAL STRETCHING OF LIQUIDS. 
slightest jar, the bubble always appearing at the contact of the solid with the wall, and 
the loose piece being generally tossed up when the rupture took place. I succeeded 
best with one small irregular bulb with a projecting stem ; this could be gently 
waved about in the stretched liquid, while the foot of the stem adhered to a point on 
the side of the containing vessel (showing incidentally that considerable currents may 
exist in a stretched liquid). The explanation of this phenomenon, which at first 
puzzled me, may, I think, be given as follows. 
At the surface of glass, any liquid, such as alcohol, which wets it, is condensed. 
Over the area of contact of the loose piece with the side of the vessel, this condensed 
film is probably somewhat thinner than elsewhere, being squeezed out (as by 
hydrostatic pressure) by the cohesive attraction between the two solids. When the 
liquid is in a state of tension, there is everywhere a demand for liquid to stretch, 
which is met by any approach of the cohering surfaces, for such approach will increase 
both the closeness and the area of contact, and yield a supply of hitherto condensed 
liquid, by diminution of the surface over which it has been condensed. Any displace¬ 
ment in the direction of further approach will therefore be resisted only by the 
elasticity of the solids, called into play by the deformation at the area of contact. 
Under ordinary circumstances such displacement is resisted also by the hydrostatic 
pressures. To understand the instability, it is only necessary to remember that, with 
a comparatively rigid substance like glass, small relative motion of the parts may 
generate very large impulsive stresses at points where the relative motion is 
prevented. 
