CLARK AND LUBS : INDICATORS FOR CULTURE MEDIA 487 



such meagre aid as a small hand spectroscope afforded we were able to 

 trace the nature of this " dichromatism" and to devise a light source 

 with which the effect may be avoided. This source is simply a bank 

 of ordinary electric lights from which the shorter wave lengths are 

 screened by a translucent paper coated with an acid solution of phenol- 

 sulphone-phthalein. With this screen fairly good measurements could 

 be made with brom-phenol blue and excellent measurements with brom- 

 cresol purple. 



To illustrate the accuracy attained we may quote two tables. Table 

 2 'summarizes some measurements made upon ordinary beef infusion 

 media, some samples of which were quite dark or else turbid from 

 addition of the acid or alkali used to bring the P H value within the range 

 of the indicator used. 



In table 2 "comparator" indicates that the determination was made 

 by the compensation method of Walpole. "Dilution" indicates that 

 the tested solution was diluted five times with distilled water before 

 measurement. "Average" deviation is the average of the positive and 

 negative deviations when the electrometric value was subtracted from 

 the colorimetric value in each case. "Mean" deviation is the average 

 of the deviations, neglecting sign. 



Table 3, which gives a few determinations made on urines is self- 

 explanatory. 



In regard to each indicator the following points may be noted: 



Thymol blue, which was previously described for use in alkaline 

 solutions, exhibits very brilliant color changes at high acidities. Al- 

 though we have made only a comparatively few determinations in the 

 acid range, the indicator seems to be reliable and promises to be useful 

 in a zone of P H for which there has been no very satisfactory indicator. 

 It is hoped that others will try it in studies of the gastric contents. It 

 should be useful for vinegars and for cultures of yeast and moulds. 



Brom-phenol blue has not proved reliable when used in turbid solu- 

 tions without a properly screened light, but for many approximate 

 measurements it is useful. One may show, for instance, that material 

 such as silage which is fermented by organisms of the bidgaricus type has 

 about the same P H as pure cultures of B. bidgaricus. 



Methyl red has given some irregular results, for instance in Dunham's 

 solutions, where frequently errors of 0.2 P H were found. In media such 

 as those used in the differentiation test of Clark and Lubs 10 methyl red 

 has been found to give excellent results. 



10 Clark, W. M. and Lubs, H. A. Journ. Infect. Diseases, 17: 160. 1915. 



