﻿of the common Surface of two Liquids. 247 



lary tube when water, or alcohol containing water, was poured on 

 it. Bede* observed the height to which water or sulphuric acid 

 ascended in capillary tubes when, above one of these liquids, pe- 

 troleum was poured into the tube, and that to which chloroform 

 ascended when water was poured over the latter. 



Thomas Youngf found that a small quantity of oil poured on 

 the surface of water in a capillary tube diminished the total 

 height. 



In these experiments, however, we have two unknown mag- 

 nitudes : — the superficial cohesion, a J2 , of the common surface 

 of the two liquids; and the marginal angle co 1<2 , which, as I shall 

 afterwards show (§ 10), is not always either 0° or 180°. The 

 product « 12 cos (o u only is determined by observing the height. 



I myself { have measured the height of drops of mercury in 

 water and hydrochloric acid, without ever being able to draw 

 further conclusions regarding the cohesion of the common sur- 

 face of those liquids. 



Guthrie §, lastly, dropped into different liquids other liquids, 

 and measured the magnitude of the drops. 



Even in the few cases on which experiments are given, theory 

 has not been proved to agree with experience — partly because, 

 beside the propositions mentioned above, and « 12 = 0° or 180°, 

 relations also were assumed between the magnitudes a„ a 2 , and 

 a 12 , which, as I shall presently show, do not altogether agree 

 with experiment. 



The error of the usually assumed relation a x — a 2 =a 12 sug- 

 gests, moreover, the simple idea that for two liquids which mix 

 in all proportions the capillary constant of the common surface 

 a 12 is =0, whilst for water and absolute alcohol it should be 

 8 — 2*5 = 5*5 milligrammes. 



Hence the proposition adduced by Poisson||, that the weight 

 raised in a capillary tube will depend only on the lower liquid, 

 is not in agreement with experience. 



Thomas Young supposed the capillary constant of the common 

 surface of two liquids to be proportional to the difference^ or 

 to the square** of the difference of their densities. Even this 

 does not agree with the experiments which will be presently 

 described (§ 10). 



* Mem. Cour. Brux. vol. xxx. p. 18? (1860). 



t JEncyc. Brit. "Cohesion," Sect. II. (1816). Young's Works, by Pea- 

 cock, vol. i. p. 463 (1855). 



t Pogg. Ann. vol. cv. p. 38 (1858). 



§ Proc. Roy. Soc. vol. xiii. p. 444, vol. xiv. p. 22 (1865). In abstract, 

 Pogg. Ann. vol. cxxxi. p. 141 (1867). 



|| Nouvelle theorie de faction capillaire, p. 142. 



f Young's Works, vol. i. p. 435 (1855). Encycl. Brit. "Cohesion," 

 Sect. II. (1816). ** Ibid. p. 463. 



S3 



