60 PHYSICAL CHARACTERS OF ORGANIC SUBSTANCES 



with potassium bichromate, the red with Congo Red. Half 

 fill the tubes with water and view against a sheet of white paper. 

 The potassium bichromate diffuses into the water quickly, the 

 Congo Red very slowly. 



(8) Another test tube contains jelly, the bottom layer of which 

 contains sodium carbonate, while the top layer does not. Pour a 

 little hydrochloric acid on the top of the jelly, it diffuses down 

 through the substance. When the layer containing sodium 

 carbonate is reached by the hydrochloric acid a reaction takes 

 place, bubbles of carbon dioxide appearing in the jelly. [Note. 

 The jelly must be made up immediately before the class, or the 

 sodium carbonate will diffuse up into the top layer of jelly, and 

 the reaction will take place immediately the hydrochloric acid 

 enters the jelly.] 



C. COAGULATION OF PROTEIN COLLOID BY HEAT. 



(o) Place small portions of fresh white of egg in two test tubes. 

 Add a little water to one : the water is absorbed by the colloid. 

 Slowly heat the other: the white of egg sets to a solid. Cool 

 and add a little water. The coagulated protein no longer absorbs 

 water, having lost its colloid structure. 



D. FORMATION OF COLLOID MEMBRANE. 



(10) Carefully drop a crystal of copper acetate into a test 

 tube containing a 2 per cent, solution of potassium ferrocyanide. 

 A double decomposition occurs as the two salts meet and the' 

 copper ferrocyanide is precipitated to form a colloidal membrane 

 around the crystal. This membrane enlarges by breaking and 

 patching so that it seems to grow. 



E. SWELLING OF VEGETABLE COLLOIDS IN WATER. 



(n) Cover with water on a sheet of glass the rectangular piece 

 of seaweed [Chondrus is very suitable] provided, being careful 

 to wet both sides. Note the expansion as with the sheet of 

 gelatin in (3) . 



(12) Add water to a dried section of seaweed under the micro- 

 scope and note the expansion that takes place. 



(13) Compare the size and shape of dry peas with those of 

 peas that have been soaked in water for 24 hours. Place six 

 soaked and six unsoaked peas in rows touching one another : 

 measure the aggregate diameter of the six in each case and 

 determine the average absolute and percentage increase in the 

 soaked peas. 



