266 EXPERIMENTAL PHYSIOLOGY 



and salts, N, N/10, N/20, N/30, N/40, N/50. Use discs in dis- 

 tilled water for control. Let them stand for several hours in these 

 solutions, then measure the size of each. Tabulate the results 

 for the whole series of all reagents. (Some members of class use 

 HC1, others H 2 SO 4 , NaCl, Na 2 SO 4 , NaOH, NH 4 OH, etc.). What 

 is the optimum for each electrolyte? Does valency have any 

 effect? Antagonistic solutions should also be tried, especially the 

 following: N/10 HC1 + N/10 NaCl (add equal quantities of N/5 

 solution of each); N/20 HC1+N/10 NaCl; N/10 NaOH + N/10 

 NaCl; N/20 NaOH+N/10 NaCl. 



Another method, perhaps better than the disc method, is 

 to arrange a series of test tubes of uniform size each containing 

 the same quantity of powdered gelatin (0.5 1 gm.). 10 c.c. of 

 the given solution is added and when the swelling has become 

 constant the height of the column of gelatin is read. 



(2) Brownian movement.^. Rub a piece of a water-colour 

 tablet in water on a slide and examine under high power both with 

 reflected and transmitted light (dark field). Can you trace the 

 path of a single particle? How would you distinguish one of these 

 colloidal particles from a typhoid bacillus, for example? Add a 

 drop of warm (liquid) gelatin to the water-colour. Result? Allow 

 the gelatin to cool and harden. Result on Brownian movement? 



(1) Filter solution of methylene blue or congo red through bone 

 charcoal. The filtrate should be colourless. Now pour through 

 the charcoal 95% alcohol. The colour should pass through. Can 

 you account for this action on the supposition of changes in the 

 electric charges on the aggregates of molecules? (2) Place suc- 

 cessive drops of different dyes on the centre of a circular filter 

 paper. Explain results. 



