8 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



takes place ; if the solution is now placed in a cylindrical 

 vessel one end of which is closed by a parchment diaphragm 

 and the whole immersed in clean water, which is renewed 

 from time to time, the sodium chloride formed by the action 

 of the hydrochloric acid on the sodium silicate will diffuse 

 through the parchment and eventually be completely re- 

 moved. The silicic acid will remain behind in the cylinder. 

 The sodium chloride in this case is the crystalloid, the silicic 

 acid the colloid. The apparatus used in the experiment is 

 known as a dialyser and the process as dialysis. 



A large amount of work has been done of recent years on 

 the chemistry of colloids. It has been shown that no very 

 marked line can be drawn between the two extremes of 

 matter in the solid insoluble condition and matter in true and 

 crystalloid solution. The following are, however, typical 

 properties of colloids : — 



(1) When examined by an instrument known as the 

 ultramicroscope, colloidal solutions are all found to 

 contain particulate matter, that is, matter in an extremely 

 divided state but still existing as separate particles. 



The ultramicroscope is an instrument whose design is 

 founded upon what is known as the ' Tyndall phenomenon.' 

 The lighting up by a sxmbeam of the dust in the atmosphere of 

 a room is a matter of common observation. Tyndall found 

 that if a closed space was rendered ' optically empty ' by 

 smearing its sides with glycerine and allowing aU particles to 

 siubside and be caught by the glycerine, a beam of light on 

 passing through was invisible when viewed at right angles to 

 its path. On admitting a little smoke the path of the beam at 

 once became visible. The same phenomenon is observed with 

 solutions. A solution perfectly free from suspended particles 

 allows a beam of light to pass through and remain invisible. 

 On introducing a colloid substance such as gum-mastic into the 

 solution the path of the beam at once lights up. 



The application of this phenomenon to the study of 



