August 19, 1886] 
NATURE 
367 
the india-rubber is stretched. The tension of the real 
film at the surface of a drop of water remains constant, 
however much the surface is stretched, and therefore the 
drop breaks away instantly when enough of water has 
been supplied from above to feed the drop to the greatest 
volume that can hang from the particular size of tube 
which is used. 
I now put this siphon into action, gradually drawing off 
some of the water, and we find the drop gradually diminishes 
until a sudden change again occurs and it assumes the 
form we observed (Fig. 16) when J first poured in the water. 
I instantly stop the action of the siphon, and we now find 
that the great drop has two possible forms of stable equi- 
librium, with an unstable form intermediate between them. 
Fic. 24 —Water resting in two co-ax1l cylinders ; scale is represented by Fig. 23. 
4 8 P y fig 
Here is an experimental proof of this statement. With | have it again performing small vibrations about this 
the drop in its higher stable form I cause it to vibrate so as 
alternately to decrease and increase the axial length, and 
you see that when the vibrations are such as to cause the 
increase of length to reach a certain limit there is a 
sudden change to the lower stable form, and we may 
Fic. 25. 
now leave the mass performing small vibrations about | 
that lower form. I now increase these small vibrations, 
and we see that, whenever, in one of the upward (increas- 
ing) vibrations, the contraction of axial length reaches the 
limit already referred to, there is again a sudden change, 
which I promote by gently lifting with my hands, 
and the mass assumes the higher stable form, and we | 
form. 
The two positions of stable equilibrium, and the one of 
unstable intermediate between them, is a curious pecu- 
liarity of the hydrostatic problem presented by the 
water supported by india-rubber in the manner of the 
experiment. 
I have here a simple arrangement of apparatus (Figs. 
29 and 30) by which, with proper optical aids, such as a 
cathetometer and a microscope, we can make the neces- 
sary measurements on real drops of water or other liquid, 
for the purpose of determining the values of the capillary 
constants. For stability the drop hanging from the open 
tube should be just less than a hemisphere, but for con- 
snience it is shown, as in the enlarged drawing of the 
nozzle (Fig. 30), exactly hemispherical. By means of the 
‘Siphon the difference of levels, #, between the free level 
surface of the water in the vessel to which the nozzle is 
attached, and the lowest point in the drop hanging from 
the nozzle, may be varied, and corresponding measure- 
ments taken of 2 and of 7, the radius of curvature of the 
drop at its lowest point. This measurement of the curva- 
ture of the drop is easily made with somewhat close 
accuracy, by known microscopic methods. The surface- 
tension T of the liquid is calculated from the radius, 7, 
and the observed difference of levels, 2, as follows :— 
Fic. 26. 
Fics. 25 and 26.—Water resting in hollow cylinders (tubes); scale is represented by Fig. 28. 
for example, if the liquid taken be water, with a free- 
surface tension of 75 milligrammes per centimetre, and 
y = ‘05 cm., / is equal to 3 centimetres. 
Many experiments may be devised to illustrate the 
effect of surface-tension when two liquids, of which the 
surface-tensions are widely different, are brought into 
contact with each other. Thus we may place on the 
surface of a thin layer of water, wetting uniformly the 
surface of a glass plate or tray, a drop of alcohol or ether, 
and so cause the surface-tension of the liquid layer to 
become smaller in the region covered by the alcohol or 
ether. On the other hand, from a surface-layer of alco- 
