Mr. C. Tomlinson on the Cohesion-Figures of Liquids. 259 



whether liquids professedly delivered according to sample had been 

 tampered with between the delivery of the sample and the goods. 



A considerable amount of labour requires to be expended on 

 these cohesion-figures before the subject can be said to be ripe 

 for extended practical application ; and this labour, if health and 

 opportunity be granted me, I intend to bestow on this most 

 interesting subject. It will be necessary to procure many speci- 

 mens from different markets of the pure liquid, to take the co- 

 hesion-figure of each sample many times under varying tempera- 

 tures and hygrometric conditions of the air, to make drawings 

 of these figures, and to decide after repeated trials what is the 

 characteristic feature of each liquid as constantly exhibited in its 

 cohesion-figure. The next step will be to ascertain the charac- 

 teristics of mixtures of certain liquids made with a view to adul- 

 teration, and by means of cohesion-figures to enable the observer 

 to state not only whether a costly liquid be adulterated, but what 

 it is adulterated with, and the relative proportions of the adulte- 

 rated mixture. I have done all this for a moderate number of 

 liquids, but very much remains to be done before the method 

 can be considered as complete. There are also certain scientific 

 questions to be answered, such as the relations between the 

 figures of isomeric liquids. 



I am aware that some of the phenomena of films on the sur- 

 face of water have, on a few occasions, attracted the attention 

 of philosophers ; but no one, so far as I am informed, has con- 

 sidered films to differ from each other, or to be characteristic of 

 the substances which produce them. The behaviour of films has 

 generally had reference, in the labours of others, to some point 

 irrespective of the films themselves — as in the repulsion-theory 

 of B. Prevost* and the epipolic theory of Dutrochet*. 



It was many years ago observed by Ermannf that a drop of 

 sulphuric acid deposited on the clean surface of pure mercury 

 spreads out into a film. M. Dutrochet finds that many liquids 

 also form films on the surfaces of glass, metal, &c, provided they 

 be chemically clean; and he attributes all these phenomena "au 

 developpement subit et toujours de courte duree de la force epi- 

 polique centrifuge." It seems to me (with great submission to 

 so distinguished an observer as the discoverer of endosmose and 

 exosmose) that many of the phenomena described as the effects 

 of the epipolic force are simple results of cohesion and adhesion. 

 M. Dutrochet repeatedly states that his phenomena cannot be 

 produced unless the surfaces be absolutely clean. That, I am 

 quite sure, is a correct observation ; but the impurities, according 



* Quoted in M. Dutrochet's work entitled Recherches physiques sur la 

 Force Epipolique, Paris, 1842, and 2nd part, 1843. 

 f Annates de Physique de Gilbert, vol. xxxii. 



