on the Surface of Water. 421 



has been predicated of the interior parts of a liquid body only 

 and not of the whole mass " 37 . 



31. In order to produce distinct experimental results on sur- 

 face-tension, Professor Van der Mensbrugghe had to devise a 

 method by which one portion of a tensile liquid surface could be 

 separated from another portion of the same surface, so as to show- 

 variations in tension between the two portions. For this purpose 

 filaments of a silkworm's cocoon were cut into lengths of about 

 12 centimetres, and, ten or fifteen of these being laid parallel, were 

 tied at the two extremities. The bundle thus formed was bent 

 into an irregular circle, washed in alcohol and then in distilled 

 water, and flattened between the leaves of a book. The bundle 

 was now taken up by means of a clean glass rod, and placed on 

 the surface of water in such a way as to be exactly in contact 

 with it without being below the level. 



32. Let the two liquids be distilled water (whose surface-ten- 

 sion is equal to 7*3) and ether (of which the tension is 1*88) . The 

 water is contained in a large capsule, and a drop of ether is held 

 above that portion of the surface limited by the coil of fila- 

 ments; this immediately undergoes lively trepidations, and 

 tends to assume the circular form, evidently because the vapour 

 of ether diminishes the tension of the subjacent portion of sur- 

 face within the silken boundary, and this, in its turn, yields to 

 the superior traction of the portion external to it. The moment 

 the drop of ether touches the surface within the flexible contour, 

 the silk expands into a circular form ; but it as quickly con- 

 tracts, since the evaporation of the ether cools the surface and 

 so restores its contractile force. When, on the other hand, the 

 ether is deposited outside the silken boundary, this immedi- 

 ately becomes reduced in size, but expands again as the cold 

 produced by evaporation augments the contractile force of the 

 exterior portion. 



33. In this way may be explained the observation of Prevost 

 (14), that if a bit of camphor be held near the surface of water 

 that has been dusted with lycopodium, the powder is repelled 

 towards the edge of the vessel ; or, as in Biotas experiment (15), 

 if camphor be brought near a thin layer of water, this opens 

 and leaves a dry space on the support just under the camphor. 

 In such cases the water locally dissolves a small quantity of the 

 vapour of camphor, and thereby has its tension locally reduced, 

 while the contractile force of the other parts of the surface is free 

 to act. 



34. The rotations of camphor on the surface of a liquid, and 

 similar phenomena, are included in the following general propo- 



37 Nichols's ' Cyclopaedia of the Physical Sciences/ 2nd edit. 1860. Art. 

 " Liquid." 



