230 Scientific Proceedings, Royal Dublin Society. 



supposes the sap of plants to have a negligible cohesion. Ursprung tested 

 the sap (extracted from a bleeding plant) in an apparatus which is a modifi- 

 cation of that described by Tait (6), and later] by Asken.asy (1). In it a 

 porous evaporating surface draws up water, which in turn adheres to, and 

 raises, a column of mercury. The sap, collected by a steel tube, was conducted 

 by a rubber tube into a flask, and then drawn into the apparatus througli a 

 plug of cotton wool. For anyone wlio has experimented on the cohesion of 

 liquids it will not be surprising tliat tension was not establislied in sap 

 collected and introduced in this manner. One would expect by such a 

 procedure that many insufficiently wetted particles of dust would be intro- 

 duced, and that any considerable tension would be rendered impossible. 

 Notwithstanding this, Ursprung actually did observe small tensions in 

 three experiments, but these he rejects on what appear to me inadequate 

 grounds. Two he sets aside because enough sap was not introduced ; and the 

 third is discai'ded because the sap was allowed to enter through the porous 

 vessel into the tension-chamber. Witli regard to the first two, it would 

 appear that the smallest amount of sap (if without cohesion) must destroy 

 all tension ; and it is hard to admit Ursprung's objection to the third, 

 namely, that all the dissolved air was abstracted from the sap by passing 

 through the porous substance, and hence the physical nature of the sap was 



I have recently found it easy to abstract sap in considerable quantities 

 by centrifuging short lengths of decorticated stems ; and in the following I 

 have recorded some tests carried out on sap so obtained. I confess that, 

 considering our present knowledge of the tensile properties of water, both 

 containing and free from air, these experiments would have appeared to me 

 superfluous, had it not been for the publication of Ursprung's results quoted 

 above. 



The method of experimentation was the same as that described in my 

 previous paper (4) ; and it is to be noted that in the calculation of the tension 

 full allowance is made for the distortion of the glass envelope during the 

 contraction of the enclosed liquid, so that the method is not invalidated by 

 Julius Meyer's objection, quoted by Ursprung (7). 



In the first instance the sapcentrifuged from pieces of branches of Fagus 

 sylvatica, cut from about 70 feet above the level of the ground, was enclosed 

 in the tension-tube. This sap, after collection, was boiled on three successive 

 days for about one hour in order to secure the complete wetting of dust- 

 particles fortuitously contained in it. After its last boiling it was exposed 

 for twenty-four hours as a thin layer about 4 mm. deep to the air, but 

 shielded from dust. In this way it must have become practically saturated 



