292 M. Donny on the Cohesion of Liquids, 



plate suspended from a scale-beam over a vessel of water. 

 The use of this new instrument convinced me immediately 

 that there is no similitude whatever between the rupture of a 

 solid body and this mode of separating water from water. I 

 perceived plainly that such a separation was the final result 

 of a series of successive transformations undergone by that 

 portion of the liquid which is lifted up during the ascension 

 of the plate; which transformations ultimately reduce the thin- 

 nest part of that ascending liquid to so small a diameter, that 

 it gives way, even without any further exterior exertion. The 

 first experimenters, not being aware of this mode of acting, 

 considered the separation of water from water as if it were 

 similar to the rupture of a solid body; they made their calcu- 

 lations accordingly, and so doing, reduced to the lowest pro- 

 portions that very strong molecular attraction which fixed my 

 attention in Europe and Prof. Henry's in America. 



The learned Professor has proved the magnitude of this 

 molecular attraction by observations on soap-bubbles. I fol- 

 lowed quite a different course, and arrived at more extensive 

 results. 



I constantly employed liquids placed in glass tubes, whose 

 interior diameter measured from eight to ten millimetres (from 

 three-tenths to four-tenths of an English inch). In similar 

 circumstances, two distinct molecular forces are acting, — the 

 attraction of water for water, or cohesion', and the attraction 

 of water for glass, or adhesion. In my experiments, both co- 

 hesion and adhesion appeared very weak when the liquid was 

 not deprived of that portion of air which it usually contains ; 

 and, on the contrary, proved very powerful when air was ex- 

 cluded. 



In order to exhibit this power of attraction in airless liquids*, 

 I have made use of two different disjunctive forces; that of 

 mechanical traction in my first two experiments, and that of 

 repulsive caloric in the other. 



My first experiment was made on sulphuric acid deprived 

 of air by means of a very powerful air-pumpf. In that case 

 molecular attraction proved to be superior to the weight of a 

 column of acid, whose height was 1250 millimetres (more than 

 4> English feet). 



* By airless liquids, I mean liquids deprived of air by one of the pecu- 

 liar processes described in my memoir. In this sense, distilled water, al- 

 though containing less air than common water, is far from being an airless 

 liquid. 



•f- This pump, constructed on a new plan, without either cock or valve, 

 was described in 1841. It is recorded in the Rapport du Jury et Docu- 

 ments de r Exposition de l' 'Industrie Beige en 1841, p. 161. 



