594 PLATEAU ON THE PHiENOMENA OF A FREE LIQUID MASS 



fore see that they will only diminish, by the same quantity, all 

 the pressures exerted by the mass of oil upon itself; conse- 

 quently, as all the differences remain equal to each other, the 

 condition of equilibrium will still be satisfied as regards that 

 mass. It is evident that the same mode of reasoning may be 

 applied to the pressures exerted by the surrounding liquid upon 

 itself, pressures which will retain their directions, all of which 

 will only be diminished to the same extent by the attractions 

 emanating from the oil ; so that the condition of equilibrium will 

 still be satisfied as regards the hollow figure of the surrounding 

 liquid. Thus the whole of the molecular actions due to the pre- 

 sence of the surrounding liquid will not tend in any way to 

 modify the figure of equilibrium of the immersed mass, which 

 figure will consequently be identically the same as if that mass 

 were really void of gravity and were placed in vacuo. We can 

 therefore leave the surrounding Uquid completely out of the 

 question, its sole function being to neutralize the action of gravity 

 upon the mass forming the object of the experiments. 



9. We shall now pass to the experimental part. And first, to 

 avoid useless repetition, we shall say a few words relative to 

 the apparatus to be used. As the liquid alvvays consists of a 

 mass of oil immersed in an alcoholic mixture of the same density 

 as itself, our solid systems will all consist of iron ; and this for 

 the following reasons. In ordinary circumstances oil contracts, 

 I believe, perfect adhesion with all solids ; but this is not 

 exactly the case when the same oil is plunged into a mixture of 

 water and alcohol ; for then, in the case of certain solids, as 

 e. g. glass, the phaenomena of adhesion sometimes undergo 

 modifications which give rise to trouble in the experiments. We 

 shall meet with an instance of this in the subsequent parts of 

 this memoir. Now the metals do not present this inconvenience ; 

 moreover, the form which we have given to most of our solid 

 systems would render their construction of any other substance 

 besides a metal difficult. Now among metals we prefer iron, 

 not copper, because oil removes nothing from iron, whilst by 

 prolonged contact with copper it slightly attacks it, acquires a 

 green colour, and increases in density, which is a great incon- 

 venience*. 



* In a letter which Dr. Faraday did me the honour of sending to nie, re- 

 garding the preceding memoir, he informed me, that when about to repeat mj' 

 experiments before n numerous audience, wishing lo produce a still greater 



