Liquids on the Surface of Water, 37 



greatly reduced. The mere contact of the finger with the sur- 

 face is sufficient to lower the tension from 7'3 to 4*5. The ad- 

 dition of 1 per cent, by weight of soap to the water lowers its 

 tension to about 3, so that on such a surface the effects described 

 cannot be produced. 



12. Similar effects to those obtained with ether may be ob- 

 served with alcohol, only less marked, since the difference a — b 

 is actually less. Wood-spirit, acetic ether, paraffin oil, &c. exert 

 also about the same degree of energy as alcohol. With turpen- 

 tine the effects are still less marked, that liquid being much less 

 volatile than ether or alcohol, and but little soluble in water. 



13. When A is olive-oil (a = 3-5) and B ether, — * = 0'462, 



there is motion on the surface, as is evident from the apparent 

 repulsion of dust-particles as soon as the ether-tube is adjusted. 

 If the oil be in the form of a layer about 1 millim. thick, the 

 approach of the ether will cause it to fly open (10). With olive- 

 oil and alcohol, a — b = l, =0*29, a similar effect is pro- 

 duced, although the difference between the tensions of the two 

 liquids is only 1 ; whereas vapour of ether acting on a mixture of 

 seven parts of water and one of ether produced no effect, although 

 in such case a— 6 = 2*12. The author explains this apparent 

 anomaly by referring, first, to the difficulty vapour of ether has 

 in dissolving in such a mixture, whilst alcohol is very soluble in 

 the oil ; and secondly, the superficial viscosity (or the greater or 

 less difficulty the superficial molecules have in being displaced) 

 is greater in the case of the ether solution than in that of oil of 

 olives. A solution of carbonate of soda (a=4*28), according to 

 Plateau 12 , has a great superficial viscosity, and on this account 



12 See Phil. Mag. December 1869. The solutions used by Plateau in 

 his ingenious experiments for determining the superficial viscosity were, of 

 course, only saturated. In supersaturated solutions the superficial viscosity 

 is one of the most marked features. As soon as the flask is closed or co- 

 vered, the surface of the solution becomes quite viscous; and this increases 

 during the cooling — so that, on gently shaking or inclining the flask, the 

 superficial layer breaks up and partially mingles with the lower strata, like 

 a strong syrup mingling with water. When along and wide tube contain- 

 ing a strong saline solution is watched during the cooling, it is seen 

 that the superficial layer is constantly being dissolved and as constantly 

 being renewed, the one by contact with the hot liquid beneath, which 

 causes the viscous layer to descend in saccharine- looking streams, the other 

 by contact with the outer air, into which the surface-layer evaporates and 

 becomes viscid preparatory to crystallization. If the tube remain uncovered, 

 crystals begin to form on the surface, the air supplying the nuclei. If 

 covered, in time the solution will cool down without depositing crystals, 

 and the surface will long remain viscid. In my " Experimental Examina- 

 tion of the so-called Storm-glass" (Phil. Mag. for August 1863), many of 

 the phenomena which accompany the cooling of saline solutions are given. 



