64 EXPERIMENTAL OBSERVATIONS ON 



Could the two acids be obtained in pure solution the deter- 

 mination of their relative effective acidity could easily be made 

 by measuring their relative conductivities ; but as the physio- 

 logical chemist has to deal with them, as, for example, in the 

 gastric juice, they are present in solution with inorganic salts of 

 high conductivity, such as sodium chloride, and hence the electrical 

 conductivity method fails. 



It is just here that the method of determining concentration 

 of hydrogen ions and corresponding effective acidity, first suggested 

 by Ostwald and carried out experimentally by F. A. Hoffmann, 

 becomes of such immense value in enabling a determination of 

 this important point to be made in gastric juice. 



The best substratum to employ is methyl -acetate, and in 

 using the method recently in a large number of pathological 

 cases for the determination of the amount, and, by means of 

 control with ordinary titration methods, the nature of the acid 

 in the gastric contents, the writer has found it a most reliable 

 method. 



The recognition of the fact that it is not the total amount of 

 acid or alkali in a secretion or body fluid, as shown by titration 

 with an indicator, which confers upon the fluid its activity or modi- 

 fies its activity as a catalyst, or as an active agent upon living 

 cells, but rather its effective concentration in hydrogen or hydroxyl 

 ions, is of the highest importance, and the supplying of methods 

 for determining such factors, of which an example has been shown 

 above, is one of the most important services that physical chemistry 

 has rendered to biology. 



On passing from the action of the simpler catalysts, such as 

 acids, alkalies, and inorganic salts, to the enzymes, we find that 

 the disturbing elements, of which we have had some evidences 

 above in the action of the negative ion, and of neutral salts, in 

 effecting the catalytic power of the hydrogen ion, become more 

 predominating, and often, especially at certain stages in the 

 reaction, the velocity does not experimentally obey the logarithmic 

 law at all, although the reaction is quite clearly a mono-mole- 

 cular one. 



As a result of measurements with different strengths of solu- 

 tions and of enzyme, and of the experiment only being carried 

 through the earlier stages by some observers, while others 

 carried on observations until the reaction came to a stand- 



