ON COHESION. 479 



sphere being diminished by the weight of the water, which is raised between 

 them to a greater height than on the opposite sides. But when a body, 

 surrounded by a depression, approaches another, which is surrounded by 

 an elevation, they seem to repel each other, the pressure of the water 

 urging the one, and that of the atmosphere the other, in opposite directions. 

 (Plate XXXIX. Fig. 543.) 



If two smooth plates of any kind are perfectly wetted by a fluid, and 

 brought into contact, they exhibit an appearance of cohesion, which is so 

 much the greater as the quantity of fluid is smaller : if we attempt to 

 separate them, the fluid is drawn inwards, so as to have its surface made 

 concave, and it resists the separation of the plates with a certain force, 

 which acts with a hydrostatic advantage so much the greater, as their 

 distance is smaller, and hence produces the appearance of a cohesion 

 varying in proportion to the distance. (Plate XXXIX. Fig. 544.) 



Supposing the two plates to be separated at one end, and the fluid 

 between them to assume the form of a drop, one of the marginal surfaces 

 of the drop, being narrower than the other, will act. with a greater advan- 

 tage, like a tube of smaller diameter, and will tend to draw the drop 

 towards it ; and the apparent attraction towards the line of contact of the 

 glasses will increase in proportion as the square of the distance decreases. 

 This result was experimentally observed almost a century ago, but it has 

 been usually explained on mistaken grounds. (Plate XXXIX. Fig. 545.) 



The attractive power of water being greater than that of oils, a small 

 portion of oil thrown on water is caused to spread on it with great rapidity 

 by means of the force of cohesion ; for it does not appear that any want 

 of chemical affinity between the substances concerned, diminishes their 

 cohesive power ; water readily adheres to tallow when solid, and probably 

 essential oils would adhere still more readily to ice. There is, however 

 some difficulty in understanding how these oils can so suddenly come 

 within the limit of the cohesive force of water, while the drops of water 

 themselves sometimes remain for a few seconds beyond it. 



A sponge affords us a familiar instance of the application of capillary 

 attraction to useful purposes ; it is well known that in order to its speedy 

 operation, it requires to be previously moistened, by the assistance of a 

 little pressure, otherwise it exhibits the same appearance of repulsion that 

 is observable in many other cases where the contact is imperfect. The 

 absorption of moisture by sugar depends on the same principle, and 

 here the tubes are so minute, that the height of ascent appears to be almost 

 unlimited. 



The magnitude of the cohesion between fluids and solids, as well as of 

 the particles of fluids with each other, is more directly shown by an ex- 

 periment on the continuance of a column of mercury, in the tube of a 

 barometer, at a height considerably greater than that at which it usually 

 stands, on account of the pressure of the atmosphere. If the mercury has 

 been well boiled in the tube, it may be made to remain in contact with the 

 closed end, at the height of 70 inches or more ; and by agitation only it 

 may be made to cohere so strongly as to occupy the whole length of the 

 tube of a common barometer, which is several inches more than the height 



