PLANT PHYSIOLOGY 203 



of the behaviour of proteins in the presence of various electro- 

 lytes have been carried out at definite hydrion concentrations. 

 In previous work care had only been taken to act upon equal 

 amounts of protein with chemically equivalent amounts of 

 different salts, and the resulting generalisations, such as the 

 Hofmeister series in which cations or anions could be arranged 

 in order of the magnitude of the effect they produced upon the 

 physical properties of the protein, seemed purely arbitrary from 

 the standpoint of fundamental chemical principles. 



Loeb's work has been carried out upon gelatin, which has 

 proved exceedingly suitable experimentally, as the gelatin can 

 at any time be brought into solution by raising the temperature, 

 and then the pH of the solution determined by the now well- 

 known indicator methods (see Clark, loc. cit.). 



The first result of working under conditions thus controlled 

 was to show that gelatin, as every other amphoteric protein, 

 had a definite pH value at which it appeared to be perfectly 

 neutral. 



At this value, pH 4-7, it combined neither with acid nor 

 basic ions, and such iso-electric protein readily came out of 

 solution (see also Lloyd, Biochem. Journ., 14, pp. 147, 170, and 

 584-5, 1920). In more acid solution, i.e. with pH<4-7, 

 gelatin could only behave as a base and combine with anions 

 whilst in more basic solution, with pH > 47, it could only 

 behave as an acid and combine with cations. 



This behaviour of gelatin was beautifully illustrated in an 



experiment of Loeb's in which equal quantities of gelatin were 



soaked in different strengths of nitric acid, then washed with 



M 

 cold water to remove excess acid and transferred to ^r- silver 



64 



nitrate. 



The gelatin samples were then all thoroughly washed with 

 ice-cold water to remove any silver nitrate not combined with 

 the gelatin. B}^ melting the gelatin masses, solutions were 

 obtained from samples of which the pH could readily be deter- 

 mined, whilst by simply standing the gelatin tubes in the light, 

 the darkening of the silver soon showed where silver had been 

 retained by the gelatin. It was then clearly demonstrated that 

 silver was present only in those tubes in which the pH was 

 > 4*7. Thus the gelatin only combined with the Ag+ cation 

 when it was in contact with it at a pH more alkaline than its 

 own iso-electric point. 



Very significant for the cytologist also are the results of 

 experiments with acid and basic dyes. Basic dyes are only 

 retained by gelatin, after thorough washing with cold water, in 

 solutions with pH > 4-7 ; acid dyes are only retained in solutions 

 with pH < 47. 



