igo2] RISE OF THE TRANSPIRATION STREAM 27 I 



in many of its branches, and as the field of labor is divided, we 

 turn to the physicist for skill in the solution of such purely 

 mechanical questions as this must be. But while as keen a 

 worker as Schwendener can find no physical ground for even the 

 existence of a flow past bubbles, we have only ourselves to rely 

 on for an explanation. The soundness -and thoroughness of 

 physiology is usually measured by its nearness to physics and 

 chemistry; but in the present state of physical knowledge on 

 this point no theory can rest on it and be acceptable. The 

 problem is distinctly outside of my province, and the experi- 

 ments I have made on it were for the personal satisfaction of 

 feeling a new field, without any thought of clearing it. I will 

 speak of only two or three of them. 



As stated before, Vesque's experiments in glass tubes are easy 

 to repeat. If a coiled hair is pushed into a wet tube it straightens 

 in part, but will hold up an obstruction such as a bit of copper 

 slid down from above ; and this will afford considerable opposi- 

 tion to the rise of a bubble let in from below ; while the hair 

 runs down the side of a tube from the water above to that below 

 the bubble. With care, bubbles can sometimes be held in place 

 by the hair alone, but not as certainly. I have found it con- 

 venient to set the tube, so fitted up, in a vessel of water on the 

 top of a microscope ; the focusing adjustments of the micro- 

 scope served to elevate or lower the vessel. Measurements were 

 with a cathetometer. The meniscus in the tube being well above 

 the obstruction to the bubble, the vessel was slowly raised ; the 

 bubble was stationary, while the meniscus higher up slowly rose. 

 Nothing else was to be expected ; this need show only that the 

 surface tension in the angles formed by the hair and the wall is 

 sufficient to hold in the angle water which connects that above 

 the bubble with that below it. 



In a Jamin's chain, the bubbles moving with the water, and no 

 water passing them, capillarity will hold the highest meniscus an 

 indefinite height up in the tube. If Vesque's suggestion, that an 

 atmosphere should lift in the wood a total of 10"* of water, 

 exclusive of bubbles, is tenable, the height to which the highest 



