178 * INTRODUCTION TO GENERAL PHYSIOLOGY 



Arsenious sulphide is made by passing a current of hydrogen 

 sulphide through a saturated solution of arsenious acid in distilled 

 water and allowing the coarse particles to deposit. It may, with 

 advantage, be dialysed to remove the dissolved gas. The process 

 is described below. 



Ferric hydroxide is made by taking a strong solution of ferric 

 chloride (or better, ferric acetate, as used by Graham, on account 

 of the greater hydrolytic dissociation of the salt of the weak acid). 

 Place in a dialyser made by tying a piece of wet parchment paper 

 over the wide end of a bell glass, such as used for osmotic ex- 

 periments previously, but larger. Repeated changes of distilled 

 water on the outside remove most of the hydrochloric or acetic 

 acid formed by the hydrolysis of the salt. Colloidal ferric hydrox- 

 ide may also be made by dissolving precipitated ferric hydroxide 

 in ferric chloride and then dialysing to remove excess of electro- 

 lytes. The " solution of dialysed iron " of the " Pharmacopoeia " 

 may serve also, but it is better to prepare it. 



Action of Electrolytes. Make solutions of: 



Potassium sulphate -o.i molar in K" (0.88 per cent.). 

 Calcium sulphate -o.oi molar in Ca" (0.173 P 61 " cent, of gypsum). 

 Lanthanum sulphate-o.ooi molar in La"' (0.0364 per cent, of the 

 cryst. salt). 



Add equal volumes of each to three samples of arsenious sulphide. 

 The precipitating power is about the same. Potassium sulphate 

 in o.ooi molar solution has no effect. Since the lanthanum solution 

 is much more effective than the potassium, although the SO 4 " ion 

 is in only r^th the concentration, it is clear that it is the cation 

 (positive) that is the active one, and the greater power of the 

 bivalent and trivalent ions is obvious. Thus, a negative colloid 

 is precipitated by a positive ion. 



A similar series of experiments may be made by precipitating 

 ferric hydroxide with the following solutions : 



Sodium chloride -o.i molar in Cl' (0.585 per cent.). 

 Sodium sulphate -o.oi molar in SO 4 "(o.33 per cent, of the cryst. salt). 

 Sodium phosphate -o.oo i molar in PO/" (0.0138 per cent of the acid 

 phosphate). 



The phosphate solution is made neutral to neutral red by adding 

 sodium hydroxide. These three solutions will be about equal 

 in precipitating power. Thus the electro-positive colloid is aggre- 

 gated by anions. 



The sign of the charge on kaolin in suspension may be dcter- 

 mfncd by testing with the two series of salts. It will be found 

 to be electro-negative. 



