1894 - 95 .] 
Mr J. B. Hannay on Drops. 
443 
oil — i.e. it is connected with the motion of the forming drop. We 
saw that the viscosity of the oil buoyed up the water drop, and so 
caused undue increase in the size of the drop, independent of its 
increase through the neck. In the case of mercury in air, the time 
of the rupture of the neck actually varies, the shape and size of the 
ripe drop being always the same. The reason of the varying life- 
time of the neck is found in the downward velocity of the parting 
drop. When the rate is very slow the mercury composing the drop 
starts to fall from rest, and the rate of the closing of the neck in this 
case is entirely controlled by the contractility or surface tension 3 
but as the rate increases, the mercury is already in downward motion 
at the moment of its beginning to part, so that its velocity of descent 
is greater the quicker the rate, and it actually tears the drop away 
from its root, and closes the neck more quickly than would result 
from the action of the contractile power of the liquid 3 and the great 
density of the mercury emphasises this action. 
The effect of gravity on the formation of drops can be very 
prettily illustrated when dropping water in oil by introducing (by 
means of a small pipette with a capillary delivery point) some dense 
solution into the interior of the drop, when a small drop will form 
and detach itself (as shown in fig. 3, a, h and c). 
The conclusions at which I have arrived by an examination of 
the closing neck are similar to those held by Tate {Phil. Mag. 
1864), who concludes that “ the weight of the drop is sensibly pro- 
portional to the diameter of the tube from which it falls, hence the 
force which holds the drop is a surface one, and not one oj general 
cohesion.” 
This is clearly shown by comparing the weights at which the 
neck of a water-drop is ruptured when dropping in air or in oil from 
the same tube. Taking a wide tube, the weight in air is 0T78 
grm. In oil the neck is ruptured by a weight of only 0*052 grm., 
or about one-third. The drop, however, has a volume of 0*585 
C.C., or fully three times that of water in air. In this case, the 
material of the neck is identical, and the interior water is in no 
way affected 3 while the neck of the water-in-oil drop is wider than 
that of the water-in-air drop, yet the force required to produce the 
conditions for rupture is only one-third. The only change in the 
condition is that of the outer skin of the drop. Had the liquid any 
