TRANSPIRATION AND ASCENT OF SAP DIXON. 417 



It is instructive to note how the cohesion of the water in these 

 experiments is overcome. The rupture starts as an extremely small 

 space or discontinuity in the water. Immediately surface tension 

 forces develop at the surface of this bubble. At its inception, being 

 extremely small, these forces are very great, but if the bubble en- 

 larges, the surface tension forces tending to close it rapidly diminish. 

 In our experiments the forces tending to open it are (1) the momen- 

 tum of the water conferred on it by the shock, and (2) the gravita- 

 tional pull giving rise to the tension in the liquid. We may neglect 

 the vapor pressure of the bubble, as it is balanced by the vapor in the 

 other limb. If the break opened by the shock is so small that its sur- 

 face tension forces can withstand the tension in the liquid the bubble 

 will close again; but if once the bubble formed is so large that its 

 surface tension is overcome by the tension of the liquid, an unstable 

 condition is entered on, and the bubble is continually enlarged till 

 the tension of the liquid is nil. It is, however, evident that if at 

 any moment we could confine the bubble and prevent it from enlarg- 

 ing, the liquid would again pass into a state of tension due to the 

 weight of the lower parts. 



Quite recently the author 1 has been able, by using Berthelot's 

 method, to show that the cohesion of water amounts at least to 

 150 atmospheres and that water, even when subjected to a tension 

 of this magnitude, refuses to be severed from the walls of the con- 

 ducting tracts of plants. The water used in these experiments was 

 saturated with air and contained in it pieces of the conducting tracts 

 of plants. The range of temperature over which this cohesion was 

 exhibited lay between 25° and 80° C. 



The theory of the ascent of sap, which Dr. Joly and the author 

 advocate, assumes that the water in the conducting tracts of high 

 trees hangs there by virtue of its cohesion just in the same way as 

 the water hangs in the J tube. The adhesion of water to the walls 

 of the tracheae may be shown to be very great. Thus, if a fresh 

 piece of wood from the conducting tracts is inclosed in a vessel filled 

 with water in a state of tension, it will be found that in every case 

 rupture will occur at the surface of the glass rather than at the 

 walls of the tracheae. The adhesion of water to the walls of the 

 conducting tubes is thus probably always greater than the adhesion 

 of water to glass. This is quite to be expected, if we take into account 

 the manner in which water permeates the substance of the walls of 

 the trachese when it is brought into contact with it. 



The teaching of all these experiments is obviously that water 

 under suitable conditions can transmit a pull just like a rigid solid. 

 In the liquid, however, the stress is hydrostatic, and, like hydrostatic 



1 H. H. Dixon, Note on tensile Strength of Water : Proc. Roy. Dublin Soc, 1909, 

 and Notes from the Botanical School, Trinity College, Dublin, vol. 2, No. 1. 1909. 



97578°— sm 1910 27 



