DIFFUSION AND OSMOSIS 51 



some of the salts from the soil before being absorbed by the 

 plant. Pure water poured over the heart of a delicate plant 

 may burst its cells owing to its low osmotic pressure. In many 

 medical and surgical applications, on the other hand, a low 

 osmotic pressure is of advantage. Thus, in order to remove 

 the dry crusts of eczema and impetigo, the most efficacious 

 application is a compress of cotton wool soaked in warm 

 distilled water. Under the influence of such a hypotonic 

 solution the dry cells rapidly swell up, burst, and are 

 dissolved. 



Cooking is also very much a question of osmotic pressure. 

 If salt is put into the water in which potatoes and other 

 vegetables are boiled, osmosis is set up and a current of water 

 passes from the vegetable cells to the salt water. The cellular 

 tissue of the vegetable becomes contracted and dried, and the 

 membranes become adherent, the vegetable loses weight and 

 becomes difficult of digestion, in consequence of its hard and 

 waxy consistency, which prevents the action of the digestive 

 juices. Vegetables should be cooked in soft water, and should 

 be salted after cooking. When so treated, a potato absorbs 

 water, the cells swell up, the skin bursts, the grains of starch 

 also swell up and burst, and the pulp becomes more friable. 

 The digestive juice is thus able to penetrate the different parts 

 of the vegetable rapidly, and digestion is facilitated. Anv one 

 can easily prove for himself that a potato boiled in salt water 

 diminishes in weight, whilst its weight increases when it is 

 cooked in soft water. 



The method of cryoscopy is also of considerable service in 

 forensic medicine. As shown by Carrara, the cryoscopy of the 

 blood is an important aid in determining the question whether 

 a body found in the water was thrown in before or after death. 

 In the former case the concentration of the blood will be much 

 diminished. In certain experiments on dogs the cryoscopic 

 examination of the blood showed a freezing point of — '6° C. 

 The dog was then drowned, when the freezing point of the 

 blood in the left ventricle was increased to — '29° C, and that 

 in the right ventricle to — '42° C. On the other hand, when 

 a dog was killed before being thrown into the water, the 



