1894-95.] 
Mr J. B. Hannay on Drops. 
437 
On Drops. By J. B. Hannay. 
(Read May 6, 1895.) 
The formation of drops, their variation with density and chemical 
composition of the liquid forming them, and their variation with 
temperature and frequency, have been the subject of investigation 
by several chemists and physicists, yet investigators are by no 
means at one as to the definition of a drop or the cause of its 
parting from the flowing liquid. 
The first investigation by which it was sought to find a definition 
of a standard drop, and to discover the laws regulating its formation, 
was that carried out by Guthrie,* who was, however, driven to the 
conclusion that no perfect drop exists, but that every drop is more 
or less imperfect. It is well known that the quicker the rate of 
dropping the larger is the size of the delivered drop ; and Guthrie 
explained this by supposing that when a drop parts from a solid sup- 
port, a portion of the root or stem of the drop is torn back by the 
attraction of the solid from which the drop falls. Guthrie allowed 
his drops to fall from a rounded surface, and he supposed that when 
the flowing of the liquid to the solid was quick the attraction of 
the solid for the liquid was more fully satisfied, and hence a smaller 
portion of the root of the drop was torn back to satisfy the attrac- 
tion of the solid. 
According to Guthrie, a full normal drop could not exist, all drops 
being more or less imperfect. 
This conclusion may be tested by making a liquid drop from it- 
self, by which the question of the attraction of a solid is entirely 
eliminated, and this may be done in two ways. The first is to ascer- 
tain whether or not there is a variation in the size of the drops into 
which a smooth running stream divides as it falls, when the rate is 
varied. This is a case of a liquid entirely dissociated from any 
solid. The second method is to allow a liquid to drop from a column 
of its own substance retained in position by retaining walls to 
which the liquid does not adhere. This second condition may be 
easily attained by using mercury as the liquid, and dropping it from 
* “On Drops,” Froc. Roij. Soc.^ vol. xiii. 
