174 



PHYSIOLOGY OF NUTRITION. 



To demonstrate clearly the fact, which is of great importance 

 in explaining the above experiments, that 

 water retained by capillarity can hold itself 

 in equilibrium against not inconsiderable 

 pressures, we make the following experi- 

 ment, using the apparatus depicted in Fig. 

 62. The shorter limb of a bent glass tube, 

 a few millimetres in diameter, is drawn out 

 at its end to a fine capillary. We now 

 place the tube in a glass cylinder filled 

 with water, with the capillary opening a 

 few centimetres below the surface of the 

 water, and pour mercury into the longer 

 limb of the tube till a pressure of about 

 20 cm. of mercury is attained. We ob- 

 serve that a fine stream of bubbles escapes 

 from the capillary end of the tube. The 

 mercury sinks further and further in the 

 long arm of the tube till its pressure, as 

 indicated by the difference in level in the 

 two limbs, is reduced to a few centimetres, 

 and then no further depression of the 

 mercury takes place, and at the same time 

 the stream of air ceases. The fine open 

 end is blocked with water. The final 

 mercurial pressure is held in equilibrium 

 by capillary attraction of water into the drawn-out portion of the 

 tube. 



If we immerse in water the blade of a leaf of Caltha palustris, 

 or Nymphaea, or part of a leaf of Allium Cepa, and blow into 

 the leaf-stalk, or, in the case of Allium, into the open end of the 

 leaf, we succeed here again, in this simple manner, in forcing air 

 through. The surface of the immersed portion of the Allium 

 leaf exhibits a beautiful silvery lustre, due to its being covered 

 with a layer of air, which causes total reflection of light. If by 

 rubbing with the finger we remove the adherent layer of air from 

 any parts of the leaf, the water obtains access to them, and they 

 take on a green colour. Air now escapes from the leaf on blowing, 

 only where the epidermis is still provided with its silvery covering. 

 No air bubbles are set free from the wetted parts since the sto- 

 mata are here stopped up by water attracted by capillarity, and 



FIG. 62. Apparatus for 

 investigating the permea- 

 bility of capillaries to air. 



