676 wherry: the habitat of the walking fern 



of the acids (or alkalies) to persons not accustomed to thinking 

 in such terms, actual numerical intensities of acid and alkali 

 are also given. These have been obtained by subtracting 7, 

 the P H value of a neutral solution, from each P H value in turn, 

 making the sign + , and raising 10 to the power indicated; the 

 figures derived from P H values less than 7 going hi the acid, 

 and from P H values greater than 7 in the alkaline column. 



For the two preceding determinations, 2-gram samples of the 

 soils, in their natural condition, were treated in 50 cc. centrifuge 

 tubes with 10 cc. of distilled water which had been boiling for 

 some time in a resistance-glass flask to remove carbon dioxide. 

 The tubes were stoppered, allowed to stand for several hours, 

 and then centrifuged. Five cc. of the liquid, which represents 

 the soluble matter in 1 gram of soil, was titrated in a resistance- 

 glass vessel, using phenolphthalein as an indicator, carbon 

 dioxide being continually boiled out. The remainder of the 

 liquid was used for the color comparison, phenolsulfonephthalein 

 and rosolic acid being found to be the most convenient indicators. 

 The solutions of standard concentration required were tested 

 electrometrically by Dr. Gillespie, to whom the writer's hearty 

 thanks are herewith extended for this courtesy, as well as for 

 much valuable advice in connection with the methods of pro- 

 cedure. 



Because of the fact that phenolphthalein changes color only 

 in a somewhat alkaline solution, the true alkalinity is always 

 greater than the alkalinity inferred from the normality values; 

 indeed, when the solution is only weakly alkaline, as in soils 

 Xos. 3, 7, 13, 14, and 19, the two methods give apparently con- 

 flicting results, in that the reaction is acid toward phenolphthalein 

 yet alkaline when compared to a truly neutral solution. It 

 seems probable that the true alkalinity (or acidity) is of more 

 significance than the normality, with reference to the growth of 

 plants, and the normality determinations are given merely to 

 permit comparison of the results with those which have been 

 obtained in other similar investigations. 



The rocks on which the walking fern was observed to grow 

 are listed in Table 1 in order of decreasing lime content. Only 



