42 Mr. C. Tomlinson on the Motions of certain 



get a movement of rotation ; or by one single force and a couple, 

 when the soapy disk would be displaced in turning upon itself. 

 These motions do not continue long, since the soapy solution 

 gradually invades the surface of pure water external to the coil, 

 and so diminishes greatly its tension. 



23. It has already been stated (20) that the tension of water 

 covered with a film of turpentine diminishes up to a certain limit 

 in proportion as the film becomes thick. Water covered with a 

 film too thin to give a uniform white tint of the first order has a 

 tension equal to about 5 ; after a second drop of turpentine has 

 been deposited, the contractile force is only 4*71, after a third 

 4*12, after a fourth 3*7, and after a fifth 3*43. As the ten- 

 sion of turpentine is only 2'9, we have a sufficient reason why 

 several drops of turpentine in succession spread upon the sur- 

 face of water. As soon as the contractile force of the water 

 is reduced to 3, the drops of turpentine no longer spread, but 

 assume the lenticular form. I may remark, however, that the 

 phenomena of cohesion-figures are not well exhibited after the 

 deposit of the first drop. Some oils (that of lavender, for ex- 

 ample) will perform their evolutions on the film of a second drop, 

 but a third drop remains as a film. It also happens, with newly 

 distilled oils especially, that the second drop remains for a long 

 time in the spheroidal state on a film of the first, froui which it 

 is separated by a cushion of its own vapour. I beg to refer to a 

 note on this subject 16 . 



24. It will be readily understood that the great difference in 

 tension between water and the oils causes the latter to spread 

 rapidly on the surface of the former, while they maintain the 

 lenticular form if the water be covered with even an impercep- 

 tible film of greasy matter, since in such case the tension of the 

 water becomes diminished. It is a difficult question to deter- 

 mine how the lenticular form can constitute a figure of equi- 

 librium when the tension of the water on which the oil-lens 

 rests is still greater than that of the oil. Professor Van der 

 Mensbrugghe takes it for granted, although I very much doubt 

 the fact (25), that at the surface of separation of the two liquids 

 there is greater or less adhesion, which must diminish the con- 

 tractile force of each liquid, so that a resultant tension prevails 

 over such surface which is neither that of the water nor that of 

 the oil. Hence he arrives at the general condition of equilibrium 

 of a liquid lens resting on the surface of another liquid. Let t y 

 t 1 , t" be the respective tensions of the surrounding liquid, of the 

 upper face of the lens, and of the surface of separation of the 

 two liquids, and a, a', a" the angles formed with the horizon 

 at one point of the edge of the lens by tangents to the sec- 



16 Phil Mag. for December 1863. 



