92 THE THEORY OF IONS 



whole of the water. In like manner, the salt may 

 be removed from a protein-salt mixture by dialysis, 

 but all the salt cannot be removed ; the protein-ion 

 combination is therefore very stable. This again is 

 of importance in the living organism. Through the 

 decomposition of large molecules of protein or carbo- 

 hydrate during metabolism, there are produced cer- 

 tain substances, e.g. urea, sugar, etc., which are 

 of a low molecular weight, readily diffuse, and have 

 other characteristics in common with the electro- 

 lytes. But fortunately such substances are non- 

 electrolytes, they are only ionised very slightly or 

 not at all, and they have no power to displace the 

 electrolytes from the ion-protein combination. The 

 ions, and not the salts, are therefore essential to the 

 organism, and the stability of the ion-protein com- 

 pounds has a protective influence upon its well-being. 



That it is the combination of ions with the protein 

 which gives an electrical reaction ; and that it is 

 the metallic ion which has the power of precipitating 

 protein has been proved by experiment. 



When protein has been rendered as free as possible 

 from salts by dialysis against running water, thereby 

 rendering it practically ion-free, it is found to have 

 no electrical charge whatever. But native protein, 

 i.e. protein of living organisms, has a recognisable 

 negative electric charge. When positively charged 

 metallic ions (cations) are added to such a protein 

 they cause precipitation by neutralising its electrical 

 charge that is, by converting its kinetic into 

 potential energy, so that aggregates of sufficient 

 size are formed to fall to the bottom. 



If an electric current is sent through a solution of 



