THE MOLECULAR FORCES IN PLANTS. 141 



the micellae, and in this way is brought about the increase in 

 volume of the bodies. 



Wood when placed in water swells much more considerably 

 radially and circumferentially than in the direction of the axis. 

 We can easily prove that this is the case if, with a millimetre 

 scale, we take the measurements of fairly large cylindrical pieces 

 of wood, respectively when dry and when soaked with water. 

 We use for the experiment pieces of wood about 100 mm. long 

 and 80 mm. in diameter. 



Very energetic imbibition, which finally results in the complete 

 disorganisation of the molecular structure, can be induced, e.g., 

 in starch-grains by heat, acids, or alkalies. If potato-starch is 

 cautiously heated on a slide over a spirit or gas flame, care being 

 taken to replace the water lost by evaporation, a very consider- 

 able increase in the volume of the grains takes place, and at 

 about 7() C. they become swollen up into glassy masses, whose 

 outline it is difficult to make out. 



If we mount some potato-starch in a drop of water on the slide, 

 and very slowly run in potash or Sulphuric acid from the margin 

 of the cover-glass, we notice that at the commencement of the 

 action the layering of the grains becomes more distinct ; but it 

 afterwards disappears, the starch-grains undergo considerable in- 

 crease in volume, and finally swell up into glassy masses. 



When particles of water penetrate into bodies capable of im- 

 bibition, they must necessarily suffer condensation owing to the 

 strong attraction which will be exerted by the micella? upon the 

 fluid. When, however, such condensation takes place, heat is set 

 free, and, in fact, it can be shown that the process of imbibition 

 is accompanied by a rise of temperature. I have placed 100 gr. 

 of potato-starch, or 100 gr. of pea-meal, at a known temperature, 

 in a glass cylinder, and poured over it a relatively small quantity 

 of water of precisely the same temperature. It is best to run the 

 water from a burette. The temperature of the mixture at once 

 rose about 1'5 C. The rise of temperature was, however, as 

 much as 5 C. when a little water was added to potato-starch 

 which had been dried by warming and then allowed to cool. 



In the process of imbibition, however, work also is done, both 

 internally (in the separation of the micella? from one another) 

 and externally (in overcoming external resistance to the increase 

 in volume). To demonstrate in lectures that external work is 

 done I employ the apparatus drawn in Fig. 49. T Into one end 



