OSMOTIC PRESSURE 399 



4. Turgor (see Expt. 19, ii. for precautions). Take a length of sausage 

 skin parchment. Close one end tightly round a glass stopper. Fill with 

 treacle or a strong solution of sugar and then similarly close the other end. 

 Suspend horizontally in water from a loop round the middle. The ends, 

 which droop at first, giving the whole the appearance of an arch, soon begin 

 to assume a horizontal position. In a day or so the sausage skin will be 

 rigid and straight. 



5. Chemical Gardens. (1) Place 50c.c. of potassium ferrocyanide in a 

 glass jar or beaker and add a small particle of ferric chloride (small pea). 

 A semipermeable membrance of ferric ferrocyanide (Prussian blue) is 

 formed round the solid. Endosmosis occurs and peculiar growths may be 

 formed. 



(2) Add a drop of almost saturated potassium ferrocyanide from the 

 end of a glass rod to a solution of copper sulphate (bench reagent). A 

 semipermeable membrane of copper ferrocyanide is formed round the 

 drop and endosmosis takes place. This causes an increase in the concen- 

 tration of the copper sulphate immediately round the drop, and blue 

 " rootlets " may be seen descending from the drop. These are due to the 

 increased density of the sulphate. 



(3) Leduc's Growths. A small flat-sided jar, e.g. a specimen jar, is filled 

 with a 1-2 per cent, solution of gelatine to which is added just enough 

 potassium ferrocyanide to give it a pale green colour. Just before the 

 gelatine has set, a little seed made from a mixture of glucose and copper 

 sulphate is planted on the bottom of the jar. Within an hour, growth will 

 be visible and may proceed for several days. 



See Leduc, Etudes de Biophysique. I. Theorie Physico-Chimique de la 

 Vie (1910) ; II. La Biologic Synthetique (1912). 



6. Electric Endosmose. (a) The passage of water through a membrane 

 by electrical means may be observed in the preparation of a semipermeable 

 copper ferrocyanide membrane when the solutions are forced into the pores 

 of the earthenware pot by an electrical current (Expt. 2). 



(b) A clean porous pot, fitted with a manometer and a non-polarisable 

 electrode, is filled with and placed in a solution of K 2 S0 4 (0-05 per cent.). 

 A current of 2-4 volts is passed so that the electrode inside the pot is 

 cathode. Note the rise in level of the fluid inside the pot. Note also the 

 increase in the alkalinity of the fluid outside the pot. 



(c) Make a collodion test tube to fit one limb of the U-tube 

 (Fig. 9). 



(1) Fill both limbs with dilute K 2 S0 4 solution. Mark the level of the 

 fluid in both limbs and, using non-polarisable electrodes, pass a current of 

 4 volts for some time through the solution. Note that water passes towards 

 the cathode and that the cathodal fluid becomes acid. 



(2) Repeat, using tartaric acid in the collodion sac and pure water 

 outside. Test for tartaric acid. 



(3) Fill the sac with gelatine sol and leave overnight. Wash out the 

 sol and repeat the expts. 



7. Determination of the Freezing Point of Urine. Principle. The freezing 

 point of water is depressed by the addition of salts which go into true 

 solution. The magnitude of the depression termed A bears a relation to 

 the molecular concentration of the solutes and therefore to their osmotic 

 pressure. 



