COLLOIDS AND COLLOIDAL SOLUTIONS 377 



In all cases a current of water is slowly circulated through the 

 beaker or other vessel, or the dialyser may be put into several changes 

 of distilled water, e.g. : 



(a) Some litmus solution is placed with a drop or two of dilute 

 hydrochloric acid in a parchment paper dish which is allowed to float in 

 a beaker of distilled water. The litmus does not diffuse out, but the 

 hydrochloric acid passes into the surrounding water. It may be tested 

 for by silver nitrate in the presence of nitric acid. If the process of 

 dialysis be continued sufficiently long (repeated changes of water), the 

 red colour will disappear and the litmus will become blue. 



(b) The same experiment is repeated with a mixture of starch solu- 

 tion and glucose ; the former being a colloid does not diffuse out, but 

 the latter, a crystalloid, diffuses out and can be tested for in the sur- 

 rounding water by Trommer's or Fehling's test. 



(c) Egg-white solution treated in the same way does, not diffuse 

 out through a parchment paper membrane. The surrounding water, 

 if tested for protein by the xanthoproteic, Millon's and the biuret 

 reactions, will show that protein is absent. 



The globulin may be precipitated in the paper dish if the egg- 

 white solution be dialysed long enough, as it is insoluble in distilled 

 water ; it dissolves on adding a little salt. 



I 1. Tyndall Phenomenon. 



If a bright beam of light be passed through a colloidal solution 

 contained in a vessel with parallel sides and the solution be viewed 

 from the side, it will appear turbid, sometimes with a coloured sheen. 



III. Colloidal solutions are often opalescent, e.g. starch, glycogen. 

 Some are coloured and show a pseudo-fluorescence : their colour in 

 transmitted light is different to their colour in reflected light. 



IV. Colloidal solutions, especially those of natural substances, have 

 a great tendency to froth if shaken. 



V. Colloidal solutions cannot generally be filtered through filter 

 paper and behave like suspensions. 



E.g. a suspension of kaolin, prepared by shaking up kaolin with 

 water, on filtration passes through, leaving only the large particles. 

 Similarly, arsenious sulphide sol passes through filter paper. 



