ii6 



BOTANICAL GAZETTE 



[FEBRUARY 



(healthy bark about 5.10, diseased about 5.65) agree well with 

 those determined without the buffer (Ph healthy = 5 .15, diseased = 

 5.61); the latter are taken, therefore, to represent practically the 

 actual acidity in each case. This is based on the assumption that 

 if the acidity of a buffer solution is the same as that of a mixture 

 of bark, pyrogallol, and water, no change in acidity will take place 

 when the buffer is used instead of water. 



Effect of buffer solutions. — The oxidations brought about 

 by mixtures of bark, pyrogallol, and the various buffer solutions 

 are given in table VII, together with the initial Ph of these mixtures 

 and their Ph after oxidation had practically ceased. 



TABLE VII 



Oxidation by mixtures of bark and pyrogallol with various buffer 

 solutions; temperature 29-3q°c. 



The principal fact shown by the results in table VII is that the 

 Ph (4 . 29) reached by mixtures of pyrogallol, water, and either 

 healthy or diseased bark when oxidation comes to an end is not 

 sufficient to inhibit oxidation when the mixture has that Ph value 

 to begin with; in fact, a greater degree of acidity does not inhibit 

 entirely, since a healthy bark mixture with an initial Ph of 3.61 

 gave an oxidation (a mercury rise) of 0.53 cm., and a diseased 

 bark mixture with an initial Ph of 3 . 78 gave an oxidation of 

 1.82 cm. The check, bark, pyrogallol, and water gave, in the 

 former case, 2.22 cm. mercury rise, and in the latter 4.27 cm. 



It might seem from this that the acidity brought about in 

 mixtures of bark, pyrogallol, and water is not the factor which 



