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internal rupture of liquid — whenever hubbies are formed, as 
when water is boiled — but under these circumstances we have 
no means of estimating the forces which cause the internal rup- 
ture ; they are molecular in their action, and for all we know 
they may be very considerable. Having thus pointed out 
that the ease of separation of the parts of a mass of liquid 
does not even imply a want of cohesion on the part of the 
liquid, I shall now point out that we have in common 
phenomena 
Evidence of Considerable Cohesion. 
These are for the most part what are considered minor 
phenomena ; they are confined to the surface of the liquid, 
and are included under what is called “ capillarity,’ 7 or “ sur- 
face-tension.” 
The phenomena of capillarity, or surface-tension, have 
recently attracted a great deal of attention, and many im- 
portant facts concerning them have been clearly elucidated, 
some of which bear directly on my present subject. 
Of the phenomena I may instance the suspension of drops 
of water, the rising of water up small tubes, the tendency 
of bubbles to contract, and the spherical form assumed by 
small fragments of mercury. 
These phenomena and others are found to be explained 
by the fact that the surface of these liquids is always under 
a slight but constant tension, as if enclosed in a thin elastic 
membrane. 
No satisfactory explantion as to the cause of this surface- 
tension has, I believe, been as yet found ; but the fact itself 
is proved beyond all question. It is a molecular phenomenon, 
and in order to offer any explanation as to its cause it would 
be necessary to adopt some hypothesis respecting the mole- 
cular constitution of the liquid. Such an explanation 
making the surface-tension to arise from the cohesion of the 
molecules of the liquid is, I believe, possible ; but this is 
beside my present purpose, which will be completely served 
by showing that 
