SALT AND WATER IN LIFE AND GROWTH 



111 



surface of the drop. This solid film forms because gelatin 

 is rendered insoluble when tannic acid is added to it. If 

 tannic acid is mixed with gelatin, all of the gelatin becomes 

 insoluble. But in the set-up here described, only the sur- 

 face layer of the gelatin solution comes into contact with the 



Fig. 31. Old Picture of a Patient Suffering from Heart Disease 

 Exhibiting Enormous Swelling of the Abdomen 



The picture distinctly shows the enormous accumulation of water in the 

 abdominal cavity of a sufferer of heart or kidney disease in the terminal 

 stage. 



tannic acid solution; the surface alone becomes insoluble, 

 forming a solid film (Fig. 32a). 



The most important property of this film is that it permits 

 water to pass, but prevents the passage of salt, sugar, or 

 similar dissolved materials. Such a film is therefore called 



