88 TRANSPIRATION AND ASCENT OF SAP ch. 



account the manner in which water permeates the sub- 

 stance of the walls of the trachea? when brought into 

 contact with them. 



The teaching of these experiments obviously is that 

 water under suitable conditions can transmit a tension just 

 like a rigid solid. In the liquid, however, the stress is 

 hydrostatic, and, like hydrostatic pressure, is transmitted 

 equally in all directions. It is not sustained consequently 

 by a single point but affects the whole internal wetted 

 surface of the containing vessel. In another particular 

 the stressed liquid differs greatly from the stressed solid ; 

 it is much more unstable. A small flaw (i.e., a bubble) 

 in the tensile liquid rapidly spreads and almost instan- 

 taneously severs the whole column ; it matters not how 

 large the cross section of the unbroken part may be, a 

 comparatively feeble tension will tear it across. In the 

 solid a metal wire, for example on the other hand, if 

 the cross section of the unbroken part is sufficient, a 

 small discontinuity in its substance is immaterial, and the 

 stress may be successfully resisted by the intact part. 

 This difference in the behaviour of the two forms of matter 

 when submitted to a stretching force is to be referred to 

 the fact that the particles of a liquid are perfectly mobile 

 and are free to move round each other without being 

 opposed by any sensible internal forces, whereas in solids 

 there is a great opposition to the relative motion of the 

 parts. To this property solids owe their rigidity. In fact, 

 in tension experiments the liquid becomes capable of sus- 

 taining and transmitting tensile stresses only when it is 

 adhering completely to a rigid envelope which confers on 

 the liquid a pseudo -rigidity. The state of tension then 

 persists because the stretching forces act solely against 

 the cohesive properties of the liquid (i.e., in an endeavour 

 to separate the water molecules from one another a 

 separation which a liquid is able to withstand as well as 

 a solid). If, however, the liquid is free to change its 



