CAPILLAR Y A TTRA CTION. 49 



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 



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