320 THE EQUILIBRIUM OF COLLOID AND 



especially two dyads in the same periodic group of elements are 

 interchangeable. For example, one heavy metal can take the 

 place of another, and even the paradox is arrived at that the 

 poisonous action of one of these heavy metals is decreased by the 

 simultaneous presence of another, so that instead of there being 

 an additive effect of the two poisons, one balances the other and 

 protects in part from its action, so that the lethal dose of either is 

 increased. 



The ions of the inorganic salts, at the same time that they are 

 in a loose type of union with the proteins, possess a freedom of 

 movement which shows itself in their giving to the solution in 

 which they exist in common with the colloidal proteins many of 

 the more important physicochemical properties of a saline solution. 

 For example, in the case of the blood-serum, the depression of 

 freezing-point is almost the same as that of an equal amount of 

 salts dissolved in distilled water, showing that here every ion in 

 the solution has its full effect in producing osmotic pressure not- 

 withstanding its adsorption by the serum proteins. Again, the 

 electrical conductivity is practically the same as that of an isosmotic 

 solution of the saline constituents alone in distilled water, showing 

 that any adsorption which may be present does not interfere in the 

 least with the movements or velocities of the ions in the electrical 

 field. Yet there is other evidence that the salts of the serum are 

 in union of some type with the proteins, and that the amount of 

 salts in the serum as regulated by the kidney cells is dependent 

 upon the combining power of the proteins. 



One fact that gives a clear indication of this is the titration 

 value for the serum in presence of one of the more stable-coloured 

 indicators, such as methyl orange or " di-methyl." It has been 

 pointed out earlier in this article that the proteins can act either 

 as acids or bases, or, as it is termed, are amphoteric to indicators. 

 Thus, blood- serum is acid to phenolphthalei'n, and must have 

 alkali added to it to produce the pink colour denoting alkalinity ; 

 at the same time it is alkaline to methyl orange or di-methyl, and 

 requires the addition of much acid before showing the acid colour 

 of the indicator. 



The actual reaction of the serum is almost that of exact neutrality 

 in the sense of physical chemistry that is to say, the concentrations 

 of hydrogen ion and hydroxyl ion are about equal. Now although 

 it is essential that the colour of an indicator for acid and alkali 



