184 EXPERIMENTAL OBSERVATIONS ON 



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

 tion of their relative effective acidities could easily be made by 

 measuring their relative conductivities; but as the physiological 

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

 they are present in solution with inorganic salts of high conduc- 

 tivity, 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 pre- 

 dominating, 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-molecular 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 standstill, very different 

 expressions were obtained for the law of velocity of reaction of the 

 same enzyme upon the same substratum, and it was only later by 



