2o6 TRANSPIRATION AND ASCENT OF SAP ch. 



and is transmitted unaltered across the space in which 

 the pressure of the solutes is also exerted. 



In this respect the osmotic pressure acts just in the same 

 way as a number of internal supports, keeping the cell 

 turgid and preventing it from collapsing under the tension 

 of the solvent which drags the water across the cell. 



An evaporation engine. The suitability of evapo- 

 rating menisci as a mechanism for doing work may be 



illustrated by a model 

 designed by Dr. Joly 

 (see Fig. 30). A light 

 fly-wheel is delicately 

 hung in an air-tight 

 chamber (a). The short 

 limb of a J -shaped 

 glass tube (b) enters 

 the upper part of this 

 chamber from above, 

 and its end, which is 

 drawn to a fine 

 nozzle, is vertically 

 over the edge of the 

 wheel. The longer 

 limb of the tube dips 

 into a well of water 

 formed by a glass 

 vessel (c) beneath the 

 chamber. In the lower part of the chamber, beneath the 

 wheel, is a small cistern (d) also containing water, and from 

 this cistern leads a branching tube (e) which distributes the 

 water to twelve porous pots (/), some of which are seen in 

 the background. 



Evaporation from the surface of the pots removes the 

 water from the cistern. This diminishes the air pressure 

 in the air-tight chamber, and the atmosphere, pressing 

 on the surface of the water in the well beneath, urges 



Ft.;. 30. 



