PART II 



ILLUSTRATIVE EXPERIMENTS 



1. Gaseous Diffusion. Experiment on page 40. Try this first with 

 coal-gas and then with C0 2 . Soak the porous pot in water and com- 

 pare the rate of diffusion inwards of carbon-dioxide with the outwards 

 diffusion of air. What part does solubility play in diffusion through a 

 membrane 1 



2. Osmotic Pressure of Crystalloids. Preparation of a Semipermeable 

 Membrane. Take a clean porous pot such as is sold for Leclanche units. 

 Allow it to soak for a day in distilled water. Fill it with a 0-25 per cent, 

 solution of copper sulphate and immerse it in a 0-21 per cent, solution of 

 potassium ferrocyanide for a day or two. Wash thoroughly in distilled 

 water. The copper sulphate and potassium ferrocyanide meet in the porous 

 pot and a membrane of copper ferrocyanide is there formed (see Expt. 6). 

 The prepared pot may keep for years and be used many times. 



A rubber stopper with two holes should be permanently fixed in its 

 mouth with wax. Through one hole should be passed a long glass tube 

 or a U-shaped glass manometer. The other hole carries a tap funnel for 

 filling the pot. The solution to be tested should be coloured with methylene 

 blue or other dye which is easily seen. 



(1) What happens after 24 hours or so when a sugar solution is placed 

 in the pot and the pot immersed in water ? 



(2) Now add sugar to the fluid outside the pot till its concentration is 

 the same as that inside the pot and leave for the same period as before. 



(3) Increase the concentration of sugar outside and note the effect 

 on the level of fluid in the manometer. 



3. Blood Corpuscles. (1) Take three test tubes and place in one about 

 5 c.c. of water and in another a similar amount of 0-9 per cent, sodium 

 chloride, and in the third 2 per cent, sodium chloride. Prick the finger 

 with a sterile needle and add the same number of drops of blood to each 

 tube. Shake and examine the tubes (a) as to opacity and (b) as to depth 

 of colour. Take a drop of the fluid from each and examine under the 

 microscope. Measure the diameter of a number of corpuscles and average 

 those from each tube. 



(2) Add a drop of fresh blood to a drop of 0-5 per cent, sodium chloride 

 solution on a microscope slide. Place a card on the side of the microscope 

 stage and keeping both eyes open trace the projection of a corpuscle from 

 time to time or measure the diameter. 



(See also Haematocrite, Expt. 47.) 



398 



