170 ADHESION OF BODIES TO FLUIDS. 



Glass on the Wr)en a dlsk of ksg ; s vj to fttf fcur f ace f water 



surface ot wa- ° rt 



ter resists sepa- standing- at rest in a vessel of considerable extent, on endea- 

 ration with a vouring to separate it from the water we find a resistance 

 tional to° its proportional to the surface of the glass. On raising the 

 size. glass, we raise at the same time a column of water above 



Cause of this, the level of the surface, which resembles in its figure the 

 grooved wheel of a pulley. Its base extends indefinitely on 

 the surface of the level : as the column proceeds upward it 

 diminishes to about seven tenths of its height : above this it 

 enlarges, till its summit covers the surface of the disk. To 

 determine its volume, let us conceive in the plane of its 

 least diameter an interior canal, at first horizontal, after- 

 ward curved vertically as far as the level surface of the fluid, 

 and at that point resuming its horizontal direction. It is 

 easy to perceive, that, in the case of the column being in 

 equilibrium, the power owing to the capillariness of its sur- 

 face must balance the weight of the fluid in the vertical 

 branch of the canal. On raising the disk higher, the weight 

 becomes more powerful from the capillary attraction, and 

 the column separates from the disk. The weight of the co- 

 lumn of water raised in this state of equilibrium is the mea- 

 sure therefore of the resistance experienced in separating 

 the disk. If the breadth of the disk be considerable, we 

 find by analysis, that this weight is equal to that of a cy- 

 linder of water, the base of which- is equal to that of the 

 disk, and the height the product of one millimetre [0*391 

 of a line] multiplied by the square root of the number of 

 millimetres in the height to which water would rise in a tube 

 of glass one millimetre in diameter. The surface of the 

 water is a tangent to that of the disk ; but if these two sur- 

 faces cut each other, the preceding result must be multiplied 

 by the cosine of half the acute angle formed between them, 

 and divided by the square root of the cosine of the entire 

 angle. 

 Casewherethe When the fluid, instead of rising, would be depressed in 



fluid would a ca pil| ar y tube of the same materials as the disk, as mer- 



sink in acapil- . . 



lary tube. cury is in a tube of glass, the column raised by the disk 



has no longer the shape of a pulley : its base extends inde- 

 finitely on the surface of the fluid, but the column decreases 

 continually from this base, till it comes into contact with the 



disk. 



