OF CAPILLARY ATTRACTION AND THE COHESION OF FLUIDS. 423 



surface of the water, it will be found that an additional weight must 

 be applied to the opposite arm to effect a separation ; and the magni- 

 tude of this additional weight, is a precise measure of the force of 

 cohesion. 



If the water and the glass be placed in a vacuum, and then brought 

 into contact, the same effect will be found to obtain, and conse- 

 quently, the. cohesion is not produced by the pressure of the atmos- 

 phere ; hence an attraction must exist between the particles of the 

 water and the glass. 



528. EXPERIMENT 2. The constituent particles of a mass of fluid 

 are mutually attracted ; that is, they have an attraction towards 

 each other. 



According to the preceding principle, when a smooth plate of glass 

 is brought into contact with the water and gently withdrawn from it, 

 a thin stratum of fluid adheres to its lower surface ; now, if this 

 stratum of fluid be carefully weighed, it will be found that its weight 

 is much less than that which is required to detach it ; consequently, 

 an attraction necessarily exists, which would keep the stratum united 

 to the fluid in the vessel independently of its weight, and hence it is 

 inferred, that the particles are mutually attracted ; that is, they have 

 an attraction towards each other. 



529. EXPERIMENT 3. The constituent particles of a mass of mer- 

 cury have an intense mutual attraction ; that is, they are strongly 

 attracted towards each other. 



This becomes manifest from the circumstance of the smallest quan- 

 tity constantly assuming a globular form, and from the resistance 

 which it opposes to the separation of its parts. 



Another circumstance which proves the attractive principle in the 

 particles of mercury, is, that if a quantity of it be separated into a 

 great number of parts, they will all of them be spherical ; and if any 

 two of them be brought into contact, they will instantly unite, and 

 constitute a single drop of the same form which they separately 

 assumed. 



530. EXPERIMENT 4. The attractive power which is evolved between 

 the particles of glass and water, is sensible only at insensible dis- 

 tances ; that is, 'the attraction between the particles is imperceptible, 

 unless the distance between them be very small. 



This is inferred from the following circumstance, viz. whatever may 

 be the thickness of the plate of glass which is brought in contact with 

 the water, the force required to detach it is always the same. This 

 indicates that any new laminte of matter that may be added to the 



