Dixon — Vitality and Transmission of Water through Plants. 33 



osmotic pressure of the leaf-cells and the tensile strength of the transpiration 

 stream are taxed nearly to their limit. Unfortunately at present there are 

 no determinations as to the pressures in the leaf-cells of these trees during 

 transpiration, nor of the resistance of their conducting tubes to the current. 

 With regard to the tensile strength of water containing air, the highest 

 experimental value obtained for it up to the time of the publication of the 

 tension theory was, we believed, that of Dr. Joly and myself, viz., 7J 

 atmospheres.' But this, from the nature of the method, was necessarily 

 much below the actual value, and only indicated the adhesion of water to a 

 surface it incompletely wets. 



Berthelot's- experiments, so far as I am aware, have been always recently 

 quoted as applying only to water which is free from air.' Not having seen 

 the original papers, Dr. Joly and I were under this impression when we made 

 the experiments just alluded to. As a matter of fact, I now find, on looking 

 up his paper in the Annales de Chimie et de Physique, xxx., 1850, tliat 

 Berthelot at first experimented on water supersaturated with aii-. He filled a 

 strong-walled capillary glass tube with water at 28° and allowed it to cool to 18°. 

 Air was drawn into the tube during the contraction of the water as it fell in 

 temperature. The tube was then sealed at its fine-drawn end. After raising 

 the temperature again, the air was forced into solution, and when the water 

 occupied the whole space enclosed by the tube, it was again allowed to cool 

 to 18° or lower. It was then found that the slightest shock caused the air 

 dissolved in the water, up to that moment, to reappear as a bubble. Before 

 rupture relieved the tension, the water was distended by -^^6 of its volume. 

 The tension required to produce this dilatation Berthelot estimated at 50 

 atmospheres. Later on, at Eegnault's suggestion, Berthelot performed the 

 same experiment with air-free water, and obtained similar results. 



The tenacity of a water-film may also give us a minor limit for the 

 tensile strength of water. A film of soapy water in air which is only 12^/i, 

 or 12 X 10"' cms., thick is stable when stretched in a rigid frame. In this 

 position, it supports the stress of twice the surface-tension (T) of the soap- 

 solution. T = about 25 dynes per cm. Therefore a column of water 1 sq. 



, , 2 X 25 X 10' . , 



cm. across can support a tension of at least y^ dynes. An atmo- 

 sphere pressure is equivalent to 981 x 10' dynes. So we find according to 



1 Eoy. Soc. PhU. Trans., B, 1895. 



2 " Sur quelques Phenomenea de Dilatation forcee des Liq^uides." Ann. de Chimie et de Physique, 

 XXX., Ser. 2, 1850, p. 232 et seq. 



3 Text-book of Physics, Poynting and Thomson, "Properties of Matter," 2nd ed., p. 123; and 

 Ewart, Annals of Botany, 1906, p. 444 ; Worthington, Roy. Soc. Phil. Trans., A., 1892. 



SCIENT. PROC. E.D.S., VOL. XII., NO. III. G 



