6 HYDRAULICS AND ITS APPLICATIONS 



a rise in temperature, by increasing the molecular distance, diminishes 

 lnth cohesion and adhesion. 



Their relative values vary for different combinations of solid and liquid, 

 the adhesive force being greater than that of cohesion when the liquid 

 wets the solid and rin- n-rsii. Also the adhesive force, unlike that of 

 cohesion, is found to vary largely with the time of contact, increasing 

 within limits as this is increased. 



That water, under suitable conditions, may exist in a state of tensile 

 stress may be shown in many ways, and experiments l show that this may 

 attain considerable proportions. For example, Achard and Gay-Lussac 

 each obtained values of about 1,024 Ibs. per square foot, while Dufour 2 

 succeeded in raising water, in the spherical state and exposed to atmo- 

 spheric pressure, to a temperature of 356 P. without boiling. Since this 

 corresponds to a vapour pressure of 132 Ibs. per square inch, the cohesion 

 must have attained this value for rupture not to have taken place. 

 This high value is probably due to the absence of dissolved air at this 

 temperature. 



AVith water in bulk, the great difficulty in demonstrating cohesion is to 

 get it into such a state that it may be exposed to a direct tensile stress 

 over any appreciable area. 



The method of rupture, as it commonly occurs, may be likened to that 

 of a sheet of paper by a tear extending from one edge. Could the water 

 be brought into such a state that its rupture was similar to that of the 

 same sheet of paper when pulled directly into two parts, the effect of 

 cohesion would be more strikingly apparent. Such an action, however, 

 involving the simultaneous rupture over any appreciable area, is 

 extremely difficult to procure except where the water exists in the state 

 of a film, and on this account the effect of cohesion on the behaviour of 

 water in bulk is negligible. Its importance to the engineer is then 

 confined to its effects as shown in the phenomena of capillarity and surface 

 tension. 



That the idea of cohesion is not incompatible with the exceeding 

 mobility of the particles of water may be indirectly demonstrated by the 

 behaviour of two iron surface plates when brought into contact. Here, 

 if the plates are clean, the resistance to sliding of one over the other is 

 very small, while the force necessary to overcome cohesion and to separate 

 the plates by normal motion is surprisingly great. 



1 For an account of some experiments on cohesion, see a paper by Osborne Reynolds iu 

 " Proceedings of the Manchester Literary and Philosophical Society," 1880 1. 

 '* M:ix well's "Hoat." p. 259. 



