on the Adhesion between Solids and Liquids. 191 



experiment the needle is totally immersed in the liquid, and 

 under this modified condition we have to determine the time in 

 which the needle traverses the first 85°. Considering that when 

 only one face of the needle is in contact with the liquid, the other 

 being in the air, the friction, or rather the resistance to its mo- 

 tion, ought to be less than when the needle is totally immersed, 

 it might be supposed that the time occupied by the needle in 

 traversing the 85° would be less in the one case than in the 

 other. When the experiment was tried with water, the very 

 reverse of this was the result ; the needle traversed more slowly 

 on the water than in it. Many other liquids were tried, such as 

 glycerine, solutions of sodic carbonate, of potassic nitrate, of 

 calcic chloride, of albumen, of various kinds of soap; and the 

 effect was the same as with water. The reverse result was ob- 

 tained with alcohol, turpentine, olive-oil, ether, and carbonic di- 

 sulphide. M. Plateau (ably assisted by his son) found that on 

 the surface of the liquids last named the needle moved more 

 quickly than when it was submerged. 



This is not the place to show how M. Plateau, from these 

 results, seeks to demonstrate the existence in liquids of a viscosity 

 which has a different value on the surface to what it has in the 

 interior of the same liquid — nor the ingenious manner in which 

 he determines the relation of the viscosity to the surface tension, 

 describing the remarkable methods adopted by himself and other 

 physicists for measuring this last-named property. My present 

 object is to point out some experiments of my own, and certain 

 ideas suggested by them. 



My opinions respecting cohesion and adhesion in general, and 

 adhesion between solids and liquids in particular, as set forth in 

 the fourth edition of my Corso di Fisica Sperimentale, published 

 in 1868*, prevent me from agreeing with M. Plateau that the 

 phenomena described by him are simple effects of viscosity. The 

 experiments of Fusinieri, of Tomlinson, together with my own, 

 have satisfied me that very minute,* and in some cases insensible 

 causes may greatly modify the relation of the adhesion to the 

 cohesion of liquids, and that the smallest degree of impurity, not 

 only in the liquid, but in the vessel in which the experiment is 

 made, may vitiate the result. Hence, among other conditions, 

 I have strongly insisted on the cleanliness of the apparatus and 

 the purity of the substances used in the experiments, even before 

 the publication of Tomlinson's paper " On the Effects of a Chemi- 

 cally Clean Surface," published inl868f. For example, in speak- 



* The first part of this work was published in parts in 1867. 



t Philosophical Magazine for October 1868. Professor Luvini is not 

 aware that in my first paper on the Cohesion-figures of Liquids, published 

 in the Philosophical Magazine for October 1861, and in many other papers 



