1 86 BOTANICAL GAZETTE [September 



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them, were equal to their work. Its acceptance was naturally 



prompt and widespread, giving a new interest to the study of 



the cohesion of water from the physical side. Askenasy (1896: 



441) was able to draw up mercury 82*^"" and again 89. S'^"' by a 



pull resulting from evaporation from a plaster of Paris surface. 



Dixon and Joly (Trans. Roy. Soc. 1895:570), by a method 



better calculated to indicate very great tensions, demonstrated 



that columns of water could endure a longitudinal pull of more 



than seven atmospheres before breaking, and that this was pos- 



sible without absolute freedom from gas, and with bits of Taxus 



wood in 'the water. By the same method Berthelot had 



obtained negativ^e tensions, since variously estimated at 50 to 



150 atmospheres There is a resume of some other work, 



including some of Lehmann's own, in his Molccjilarphysik (I : 



243-5), ^^ which Naegeli is overlooked. Kamerling (189S; 



465) showed on theoretical grounds, the same as Naegeli s 



must have been, that in pure water ai rest the resistance to pull 



must be infinite. 



The supposed difficulty with other theories which the cohe- 

 sion theory sought to overcome was their failure to provide for 

 the transmission of really negative tensions, /. ^., tensions more 

 than one atmosphere less than the normal. So long as the ten- 

 sion remains positive there is no possibility of the exercise of 

 cohesion ; the least positive tension would itself prevent a rup- 

 ture of a column, whether of water or of water and air. But it 

 is argued that water can be lifted lo'" at most while its tension 

 remains positive; that when it is lifted higher, as in trees every- 

 where, negative tensions Tntist be present near the top. There 

 are two easy ways of determining empirically the opportunity 

 for the play of cohesion in the plant: measuring the tensions 

 actually present, and testing the result of artificially established 

 tensions. 



We have already mentioned that the tension of the air is 

 ordinarily less than one atmosphere, and of course that of the 

 contiguous water is the same. Well known to older writers, but ^ 



too little considered, this fact came strongly to the fore with v. | 



