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XXVIII.—On a Method of Determining the Cohesion of Liquids. 
By J. B. Hannay, F.C.S. (Plate XXIV.*) 
(Read 17th December 1877.) 
While working at an investigation (about to be published), relating to the 
flow of liquid through capillary tubes, it was thought that the cohesion of a 
liquid might influence the rate of its flow, and a method was sought for, which 
would enable the cohesion of any liquid to be determined ; the ordinary method 
of calculating the cohesion from observations of the flattening of a drop of the 
liquid resting upon a plane surface not being applicable to all liquids. The 
method which seemed most likely to yield results was that of observing the 
drops of a liquid so arranged that they fell from the end of a column of the 
same liquid, and accurately measuring the width of the neck of the drop at the 
instant of breaking, and the weight of the fluid which caused the rupture. It 
is easily seen that before this could be done a complete knowledge of the nature 
of drops would be required, and as the phenomenon of dropping has been made 
the subject of a long investigation by Professor GUTHRIE, an examination of his 
results was undertaken. They show that the drop increases in weight as the 
erowth-time decreases, that is, the quicker the rate of flowing the larger the 
drop. Many experiments were made, which, agreeing with Professor GUTHRIE’s 
numbers pretty closely, need not be quoted here. While, therefore, his experi- 
mental numbers may be taken as quite correct, they do not seem to prove the 
theory he has put forward in their explanation. The following is from his 
_ paper :—“ The most prominent fact is that, on the whole, the drops undergo a 
continuous diminution in weight or size as G.t. increases. To such an extent is 
this the case, that the most rapidly falling drops of the above table are nearly 
twice as heavy as the most slowly falling ones. The cause of this is probably 
to be sought for in the circumstance that when the flowing to the solid is more 
slow, the latter is covered with a thinner film of liquid, so that, as the drop 
parts, the solid reclaims by adhesion more of the root of the drop than is the 
case when the adhesion of the solid to the liquid can satisfy itself from the 
thicker film which surrounds the drop in the case of a more rapid flow.” It 
appeared that the correctness of this theory might be experimentally tested by 
the following method :—Suppose mercury dropping from a column of that 
liquid in a glass tube, of such dimensions that on being slightly inclined or 
shaken the mercury will run out, so that the glass acts only as a wall to retain 
VOL. XXVIII. PART II. ST 
