1922] 



ROSEN — A BACTERIAL DISEASE OF FOXTAIL 357 



centration have been carefully ascertained by Clark and Lubs 

 and the values given by these authors have been checked up by 

 numerous investigators. Explanation of the colorimetric method 

 may be had in Friedenthal (II, '04), Salm (II, '04), Sorensen 

 (II, '09), Clark (II, '15, '20) and Committee on Descriptive 

 Chart for 1918 (Conn, II, '19). 



STANDARDS FOR COMPARISON 



Having added a few drops of an indicator, like brom thymol 

 blue, to a tube of nutrient broth, how is the Ph value to be deter- 

 mined? The writer has already indicated one method, namely, 

 comparing the tube with the color chart given by Clark (II, '20). 

 This method for general laboratory work was found very satis- 

 factory; the error was small, particularly if the solution was not 

 highly colored; it was comparatively easy to manipulate and 

 could be used readily in the class room; and best of all, it was 

 very simple and comparatively inexpensive. The errors involved 

 in this method are: first, it is difficult to get 2 charts which ex- 

 actly agree in the shades of color for any one indicator, second, 

 any color in the solution to be tested interferes with the color of 

 the indicator. Nevertheless, in the absence of definitely known 

 standards, this method is to be preferred, in the opinion of the 

 writer, to the method advocated by the Committee on the De- 

 scriptive Chart (Conn, II, '20). This committee recognized the 

 value of the Clark and Lubs indicators but instead of using any- 

 thing for comparison, they simply recommend the following: 

 "Bring the media to such an acidity as to turn this indicator 

 (brom thymol blue) a distinct grass-green (neither yellow green 

 nor blue green)." 



It is evident from what has been said that it is desirable to 

 have some standards whose hydrogen-ion concentrations have 

 been definitely ascertained. Standards such as those recom- 

 mended by Clark and Lubs contain certain salts, such as borates, 

 phosphates, phthalates, which in the presence of certain indica- 

 tors present definite colors. (The hydrogen-ion concentration of 

 these standards having been previously determined by Clark and 

 Lubs and by others using the electrometric method, it thus be- 

 comes possible in performing a titration to obtain a series of so- 

 lutions of known hydrogen-ion concentrations which possess 

 definite colors at certain levels of hydrogen-ion concentration.) 

 The standards largely developed by Sorensen, recommended by 



