186 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



acid-albumin. Thus albumin, it will be seen, is capable of 

 forming salts and of combining both with acids and bases. 



Hydiated oxides such as aluminium hydroxide, Al2(0H)g, 

 or ferric hydroxide, Fe2(0H)g, are capable of precipitating 

 albumin from solution. This phenomenon is no doubt partly 

 physical and partly chemical ; physical in that one colloid 

 body on separating from solution tends to attract other 

 colloids by a process known as adsorption ; and chemical 

 in that the metallic hydroxide actually combines with the 

 albumin. This property by which colloidal precipitates tend 

 to carry out of solution other coUoids, especially those related 

 to albumin, finds application on the large scale in the chemical 

 precipitation of sewage and other polluted liquids. 



The well-known household cookery receipt for clarifying 

 soup, etc., by means of white of egg is an illustration of the 

 same property. 



It has been pointed out by the writer of this book and 

 others, that by carefuUy conducted precipitation, either with 

 hydrated alumina or ferric hydroxide, it is possible to remove 

 from solution all colloidal matter, and to obtain results similar 

 to those which are obtained by dialysis. The method may be 

 illustrated by the following example of what has been termed 

 the clarification test. 



Two hundred cubic centimetres of the liquid to be 

 examined, e.g., a sample of sewage (freed from grosser solid 

 matter by settlement and decantation), or a solution contain- 

 ing albumin, is treated with 2 c.c. of 10 per cent, solution of 

 iron or aluminium alum, together with 2 c.c. of a 10 per 

 cent, solution of sodium acetate, and boiled vigorously for 

 two minutes ; on coohng and filtering through filter paper 

 a crystal clear solution is obtained. By making a suitable 

 analytical estimation, e.g., of the amount of oxygen absorbed 

 from an acid solution of potassium permanganate of known 

 strength, or by boiling a known amount of the clarified 

 and unclarified liquid respectively with alkaline perman- 



