ON THE COHESION OF FLUIDS. 



655 



fiom 180°, twice the angle, of wliicli the sine 

 .is to the radius, as the a])parent cohesion of 

 each lo 446 grains ; that is, for gold 1, for 

 silver about .97, for tin .95, for lead .90, for 

 bismuth .85, for zinc .46, for copper .3C, for 

 antinion}' .29, for iron .26, and for cobalt .OC, 

 neglecting the surrounding elevation, which 

 has less effect in proportion as the surface 

 em|)loyed is larger. Geilcrt found tiie de- 

 pression of melted lead in a tube of gUiss 

 multiplied by the bore /equal to about .054. 



It would perhaps bo possible to pursue 

 these principles so far as to determine in 

 many cases tlie circumstances under which 

 a drop of any fluid would detach itself from 

 a given surface. But it is sufficient to infer, 

 from the law of the superficial cohesion of 

 fluids, that the linear dimensions of similai- 

 drops, depending from a horizontal surface, 

 must vary precisely in tiie same ratio, as the 

 heights of ascent of the respective fluids 

 against a vertical surface, or as the square 

 roots of the heights of ascent in a given tube; 

 hence the magnitudes of similar drops of dif- 

 ferent fluids must vary as the cubes of the 

 square roots of tlie heights of ascent in a tube. 

 • I have measured the heights of ascent of water 

 and of diluted spirit of wine in the same tube, 

 and 1 found them nearly as 100 to G4 : a 

 drop of water, falling from a large sphere of 

 glass, weighed 1.8 grains, a drop of the spirit 

 of wine about .85, instead of .62, which is 

 nearly the weight that would be inferred from 

 the consideration of the heights of ascent, 

 combined with that of the specific gravities. 

 We may form a conjecture respecting the 

 probable magnitude of a drop, by inquiring 

 what must be the circumfierence of the fluid, 

 that would support by its cohesion the weight 

 of a hemisphere depending from it: tliis 



must be the same as that of a tube, in which 

 the fluid would rise to the height of one third 

 of its diameter ; and the square of the dia- 

 meter must be three times as great as the ap- 

 propriate product ; or, for water .12; whence 

 the diameter would be .35, or a little more 

 than one third of an inch, and the weight 

 of the hemisphere would be 2.8 grains. If 

 more water were added internally, the cohe- 

 sion would be overcome, and the drop would 

 no longer be suspended ; but it is not easy to 

 calculate what precise quantity of water would 

 be separated with it. The form of a bubble 

 of air rising in water is determined by the 

 cohesion of the internal surface of the water, 

 exactly in the same manner as the form of a 

 drop of water in the air. The delay of a 

 bubble of air at the bottom of a vessel ap- 

 pears to be occasioned by a deficiency of 

 the pressure of the water between the air and 

 the vessel; it is nearly analogous to the ex- 

 periment of making a piece of wood remain 

 immersed in water, when perfectly in contact 

 with the bottom of the vessel containing it. 

 This experiment succeeds bov^ever far more 

 r«adily with mercury, since the capillary 

 cohesion of the mercury prevents its insinuat- 

 ing itself under the wood. 



V. OF APPARENT ATTRACTIONS AND KEPUL^ 

 SIONS. 



The apparent attraction of two floating 

 bodies, round both of which the fluid is 

 raised by cohesive attraction, is produced by 

 the excess of the atmospheric pressure On the 

 remote sides of the solids, above its pressure 

 on their neighbouring sides : or, if the expe- 

 riments are performed in a vacuum, by the 

 equivalent hydrostatic pressure or suction, de- 

 rived from the weight and the immediate cohe- 



