ON COHESION. ' 625 



force of a similar nature, the pressure of the atmosphere 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 atmo- 

 sphere 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 advantage, like a tube 

 of smaller diameter, and will tend to draw the drop towards it; and the ap- 

 parent attraction towards the line of contact of the glasses will increase in 

 proportion as the square of the distance decreases. This result was experi- 

 mentally observed almost a century ago, but it has been usually explained 

 on mistaken grounds. CPlate 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 airy want of 

 chemical affinity, between the substances concerned, diminishes their cohe- 

 sive power; water readily adheres to tallow when solid, and probably essen- 

 tial 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 some- 

 times remain for a few seconds beyond it. 



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



