1886.] on Capillary Attraction. 505 



liquid layer in that region by diminishing the percentage of alcohol 

 which it contains. 



In this shallow tray, the bottom of which is of ground glass 

 resting on white paper, so as to make the phenomena to be exhibited 

 more easily visible, there is a thin layer of water coloured deep blue 

 with aniline ; now, when I place on the water-surface a small 

 quantity of alcohol from this fine pipette, observe the effect of 

 bringing the alcohol-surface, with a surface-tension of only 25*5 

 dynes per lineal centimetre, into contact with the water-surface, 

 which has a tension of 75 dynes per lineal centimetre. See how the 

 water pulls back, as it were, all round the alcohol, forming a circular 

 ridge surrounding a hollow, or small crater, which gradually widens 

 and deepens until the glass plate is actually laid bare in the centre, 

 and the liquid is heaped up in a circular ridge around it. Similarly, 

 when I paint with a brush a streak of alcohol across the tray, we 

 find the water drawing back on each side from the portion of the tray 

 touched with the brush. Now, when I incline the glass tray, it is 

 most interesting to observe how the coloured water with its slight 

 admixture of alcohol flows down the incline — first in isolated drops, 

 afterwards joining together into narrow continuous streams. 



These and other well-known phenomena, including that interesting 

 one, " tears of strong wine," were described and explained in a 

 paper " On Certain Curious Motions Observable on the Surfaces 

 of Wine and other Alcoholic Liquors," by my brother, Prof. James 

 Thomson, read before Section A of the British Association at the 

 Glasgow meeting of 1855. 



I find that a solution containing about 25 per cent, of alcohol 

 shows the " tears " readily and well, but that they cannot at all be 

 produced if the percentage of alcohol is considerably smaller or 

 considerably greater than 25. In two of those bottles the coloured 

 solution contains respectively 1 per cent, and 90 per cent, of alcohol, 

 and in them you see it is impossible to produce the " tears " ; but 

 when I take this third bottle, in which the coloured liquid contains 

 25 per cent, of alcohol, and operate upon it, you see — there — the 

 " tears " begin to form at once. I first incline and rotate the bottle 

 so as to wet its inner surface with the liquid, and then, leaving it 

 quite still, I remove the stopper, and withdraw by means of this 

 paper tube the mixture of air and alcoholic vapour from the bottle 

 and allow fresh air to take its place. In this way I promote the 

 evaporation of alcohol from all liquid surfaces within the bottle, and 

 where the liquid is in the form of a thin film it very speedily loses 

 a great part of its alcohol. Hence the surface-tension of the thin film 

 of liquid on the interior wall of the bottle comes to have a greater and 

 greater value than the surface-tension of the mass of liquid in the 

 bottom, and where these two liquid surfaces, having different surface- 

 tensions, come together we have the phenomena of " tears." There, 

 as I hasten the evaporation, you see the horizontal ring rising up the 

 side of the bottle, and afterwards collecting into drops which slip 



