18 ON THE INTERNAL COHESION OF LIQUIDS. 



able to use mercury^ the tensions wliicli he obtained were 

 comparatively small ; and althoiigh he seems to have con- 

 sidered that greater tensions might be obtained^he mentions 

 one or two atmospheres as probably possible. It would 

 therefore appear that he had not conceived the possibility 

 of the cohesion of liquids being comparable with that of 

 solids. 



M. Donny appears to have been influenced in adopting 

 this limit to his idea of cohesion by a passage from Laplace^ 

 ' Mecanique Celeste/ Supplement au X^ livre, p. 3, which 

 he quotes. 



LaplacCj who was the first to investigate systematically 

 the phenomena of capillary attraction^ proceeded on the 

 hypothesis that the molecules of a liquid exercise attraction 

 for each other at insensible distances only ; and from this 

 assumed attraction he deduces the surface-phenomena. 

 The entire passage quoted by M. Donny is too long to 

 introduce here ; but the gist of it is comprised in the fol- 

 lowing extract : — 



'' Son expression analitique est composee de deux termes : 

 le premier, beaucoup plus grand que le second, exprime 

 I' action de la masse terminee par une surface plane ; et je 

 pense que de ce terme dependent la suspension du mercure 

 dans un tube du barometre a une hauteur deux ou trois fois 

 plus grande que celle qui est due a lapression de F atmosphere, 

 le pouvoir refringent de corp)s diaphanes, la cohesion, et ge- 

 neralement les affinites chimiques/' 



Laplace here speaks of the suspension of mercury to 60 

 or 90 inches as if it were a well-known phenomenon ; but 

 I cannot find any reference to experiments, or, indeed, any 

 further mention of the phenomenon in his memoir. 



I did not refer to Laplace in the first instance, although 

 I knew well that it is to him we are indebted for the theory 

 of surface-tension almost in the form now accepted, because 



