1894-95.] 
Mr J. B. Hannay on Drops. 
439 
related to any of the chemical or physical properties, but must also 
depend partly on the mechanics of flow. 
That the size of drops of solutions of salts in water has no con- 
nection with the viscosity or internal friction, is easily shown by 
comparing the numbers obtained by experiments on friction 
of that kind detailed in a paper published in 1879,* with the 
variation in the weight of drops of such solutions. Thus some 
solutions greatly increase the rate of flow, while others retard it ; 
but no connection is directly traceable between such rates and the 
size of the drops, as shown in the following table : — 
Salt in Solution. 
Strength. 
Time. 
Difference. 
Weight of Drop. 
Water alone 
370" 
0 " 
0*1040 grm. 
KI 
4 times normal 
243" 
- 127" 
0*1094 „ 
)) 
2 times normal 
297" 
- 73" 
0*1076 ,, expl. 
5) 
Normal 
312" 
- 58" 
0*1063 „ 
KNO 3 
) J 
340" 
- 30" 
0-1080 ,, sp. gr. 
4 K 2 SO 4 
i 9 
394" 
+ 24" 
0-1071 ,, 
iMgS04 
5 9 
490" 
4- 120 " 
0-1070 „ 
Here we see that KI and KNOg increase the rate of flow or 
diminish the internal friction, while and MgSO^ decrease the 
rate of flow ; yet the KI solution of double the normal strength and 
MgSO^ of normal strength form almost identical drops. 
It therefore became important to minutely examine the con- 
ditions under which a drop falls, especially the time taken for 
rupture, and to determine accurately what occurs during the 
rupture of the neck. It was found that to obtain constant results 
it was essential to have the most absolute freedom from impurity ; 
and the tube from which the drops fell was heated almost to its 
softening point after each change of solution, so that any grease or 
impurity which might have contaminated it was entirely destroyed. 
The method of regulating the rate of dropping was to cause the 
solution to drop at the lowest rate required by interposing a 
capillary tube in the flow, and then increasing the rate to any 
extent required by air pressure. In this way any required rate 
could be at once obtained. 
In order to study the conditions under which a drop parts from 
its root or stem, water was allowed to drop through oil of different 
* “On the Microrlieomoter,” Trans. Roy. Soc. 
