THE PHYSICAL PROPERTIES OF THE PROTEINS 27 



SECTION X. THE " GOLD NUMBER ". 



^ 



Zsigmondy has shown that colloids possess the property of in- 

 hibiting the transformation of bright red colloidal gold solutions into 

 the non-colloidal form, a change which can be brought about by the 

 addition of salts. The quantity of substance necessary to inhibit this 

 transformation varies greatly with different classes of colloids, and is 

 characteristic for each individual. Zsigmondy proposes, therefore, 

 to. regard this quantity as a definite factor for the identification of 

 individual colloids. The "gold number" he defines as " the number 

 of milligrams of a colloid which are just insufficient to prevent 10 c.c. 

 of a bright red gold solution, prepared under certain specified condi- 

 tions, from changing into violet, or nuances of violet immediately, or 

 shortly after the addition of I c.c. of a 10 per cent, salt solution". 



He divides the colloids into four classes, viz. : 



Class I. Colloids with the gold number 0*005 to o'l. This 

 includes gelatin, caseinogen, isinglass, animal glue. 



Class II. Colloids with gold number 0*1 to 10, including crystal- 

 line egg-albumin, gum-arabic, tragacanth. 



Class III. Colloids with gold number 10 to 500, including dextrin, 

 potato starch. 



Class IV. Colloids which are inactive, including silicic acid, 

 soluble glass and mucin. 



In the following table are given the gold numbers of certain 

 characteristic proteins : 



TABLE. 



Substance. Gold Number. 



Gelatin 0-005-0-01 



Russian glue 0-005-0-01 



Isinglass 0-01-0-02 



Caseinogen o-oi 



Glycoprotein 0-05-0' I 



Deutero-albumose oo 



Egg-globulin O'O2-O'O5 



Ovomucoid 0-04-0-08 



Crystallised egg-albumin 2-8 



Amorphous 0-03-0-06 



(After separation of the crystalline.) 



Merck's albumin 0-1-0-3 



Fresh egg-white 0-08-0-15 



It is of interest to note in the above table the high value for 

 crystallised egg-albumin. 



Method. 



Preparation of the Gold Solution. One hundred and twenty c.c. 

 of water are distilled through a silver condensing tube into a beaker 

 of Jena glass of 300-500 c.c. capacity. They are then heated, and 

 during the warming 2-5 c.c. of a o'6 per cent solution of hydrogen 

 gold chloride and 3-3-5 c.c. of a solution of the purest potassium car- 

 bonate (o- 1 8 normal) are added. After boiling, and whilst the mixture 

 is still hot, 3-5 c.c. of a dilute solution of formaldehyde (0*3 c.c. 

 commercial formalin in 100 c.c. H 2 O) are added. Only Jena glass 

 rods should be used for stirring. After a short time a bright red 



