418 Prof. G. Quincke on the Edge-angle and 



strongly ignited carbon ; the former exhibits, according to 

 the researches of Graham *, a very strong selective absorption, 

 the latter an absorption scarcely perceptible. 



On account of the difficult mobility of saline solutions on 

 the surface of glass, and the residual attraction of water to a 

 concentrated saline solution (here disregarded), the concen- 

 trated saline solution may certainly become partially dissolved 

 away and removed from the surface of the glass ; and this 

 solution leads directly to a theory of diffusion at the surface 

 of solid bodies. 



11. Briicke | has founded his theory of the diffusion of a 

 liquid at the surface of solids upon researches with turpentine 

 and cotton-seed oil, which were brought into contact with one 

 another in a space bounded by nearly- adjacent surfaces of 

 glass. 



According to Briicke the turpentine drives the cotton-seed 

 oil from the glass surface. The liquid filling the capillary 

 space may be divided into three films, of which the middle 

 one consists of turpentine and cotton-seed oil, and the surface 

 layers of turpentine. Whilst from the turpentine side of the 

 middle layer cotton-seed oil is continually withdrawn, and 

 turpentine continually from the cotton-seed-oil side, since the 

 turpentine of the surface-film is attracted more strongly by 

 the cotton-seed oil than by the turpentine in the vessel, some 

 of this continually wanders over into the cotton-seed oil, and 

 the volume of the latter is augmented. 



This theory of the diffusion of turpentine and cotton-seed 

 oil upon a glass surface is deduced from the supposition that 

 the turpentine drives away the cotton-seed oil from the surface 

 of glass. Briicke founds this supposition upon the experiment 

 that a small drop of cotton-seed oil placed upon a clean glass 

 plate is driven away by a neighbouring drop of turpentine. 

 The attraction of glass to turpentine is too great, relatively to 

 that of turpentine to itself, for the contact-angle between 

 these two substances to approach 180°. 



But where the turpentine meets the cotton-seed oil it drives 

 it away from itself by virtue of its greater adhesion. 



So, consequently, the experiment just described does not 

 prove tliis supposition, since in it the liquids, besides being 

 in contact with one another and with glass, are also in contact 

 with air. Nevertheless the supposition laid down as the 

 foundation of the theory is right, as my researches men- 

 tioned above (§10) show, where, without access of air, the 



* Pogg. Ann. xix. p. 139 (1830). 

 t Ibid, lviii. p. 82 (1843). 



