236 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



shift the hydrogen-ion concentration between two definite 

 values. 



The first of these figures can be determined either electro- 

 metrically, or by the more rapid and convenient colorimetric 

 method with indicators, which is based on the electrode method. 



The second may be obtained by titrating to the change 

 points of colour of two different indicators with a sufficient 

 range between them. 



The second figure is no less important than the first, for 

 while the first gives the hydrogen-ion concentration with which 

 the living organism is actually living in equilibrium, the second 

 shows the amount of production of acid or alkali by the 

 organism which is compatible with a given change in hydrogen- 

 ion concentration, and hence demonstrates the degree of 

 protection that the medium is capable of affording to the 

 organism. All living organisms are extremely sensitive, both 

 in their actual viability and also in the degree of their 

 physiological activity to small changes in hydrogen-ion 

 concentration, the growth and development and metabolic 

 activity being all profoundly affected by comparatively slight 

 changes. 



The resistance to change in ionic concentration has been 

 referred to as the " Buffer " effect by Sorensen, and 

 was called the " Reactivity " of the solution by Moore and 

 Wilson 6 to distinguish it from the " Reaction," which is quite 

 a different thing, 



Although the expression " Buffer Effect " has been used a 

 great deal in late years, few attempts have been made to 

 obtain a quantitative expression for the " Buffer Effect " or 

 " Reactivity " although it was determined for blood-serum in 

 1906 by Moore and Wilson, 5 when the interesting result was 

 obtained that it corresponds here to the total isotonicity 

 figure of the serum as determined by the freezing-point method. 



It was shown by these authors that the " Reactivity " can 



