August 19, 1921] 



SCIENCE 



153 



results and liave established a new Hgh-tem- 

 perature limit for active protoplasm in higher 

 plants, also a new endurance record. The 

 principal facts are as follows: 



1. Joints of Opuntia were observed to main- 

 tain a fair rate of enlargement when at a 

 temperature of 56.5° C, the air surrounding 

 them being at 58° C. (137° F.). 



2. Growth of young joints of Opuntia the 

 temperature of which rose to 62° C. (144° F.) 

 in an air temperature of 63° 0. (146° F.) 

 stopped and some shrinkage ensued, but growth 

 or enlargement was resumed when their tem- 

 perature fell to 50° C. 



3. The young joints which were subjected 

 to these temperatures were about 15 to 20 mm. 

 in width and 25 mm. in length, and after being 

 held at or near the record temperatures for 

 an hour or more, which was repeated in one 

 case, carried forward normal development, 

 reaching maturity at a normal average of 100 

 mm. in width and 130 mm. in length. 



4. It is to be noted that data from observa- 

 tions in which temperatures were taken from 

 the air or from water in which the roots or 

 aerial parts of plants were immersed, have 

 but little value in any estimation of the work- 

 ing temperature of active protoplasm by reason 

 of the abnormal hydration and transpiration 

 conditions introduced. These conditions as 

 well as the proportions and state of the main 

 colloidal components must determine the tem- 

 perature effects. 



D. T. MacDougal, 

 Earl B. Working 

 Desert Laboratory, 

 Tucson, Arizona 



a calculator for converting gas chain 



VOLTAGE INTO EQUIVALENT Ch+ OR 

 Ph VALUES 



In the determination of hydrogen-ion con- 

 centrations by eleetrometric methods employ- 

 ing the hydrogen electrode, the step of find- 

 ing the Ch* or pn value from the measured 

 voltage, with the aid of the working formula, 

 though not difficult, is time-consuming. The 

 extensive tables of Schmidt and Hoagland^ 



lUniv. of Cal. Pub. in Physiol., 5, 23, 1919. 



simplify the process considerably. They give, 

 in parallel columns, the voltages measured be- 

 tween a hydrogen electrode and a tenth-nor- 

 mal, and between the hydrogen electrode and 

 a normal calomel electrode, respectively. 

 "With these are given the corresponding pn, 

 Ch+ and CoH- values, respectively. If the 

 calomel electrode — because of difference in 

 concentration of its potassium chloride solu- 

 tion, for example — has a different value, 

 against the normal hydrogen electrode, from 

 those assumed in these tables, a simple com- 

 putation is necessary. 



By definition, pn^ — log Ch,, and the 

 working equation, derived from Nemst's 

 equation, shows these quantities to be linearly 

 proportional to the measured voltage. If in 

 all cases we had to deal with a single imvary- 

 ing reference potential, the simplest proced- 

 ure would be to draw the straight line, ex- 

 pressing the relationship, on a chart of rect- 

 angular coordinates, and to use this as the con- 

 version chart. This plan, however, is not 

 practicable in its application to all cases, be- 

 cause of the preferences of different workers 

 for different types of reference electrodes.^ 

 Some prefer thei tenth-normal, others the nor- 

 mal, still others the saturated type. In any 

 given type, there are likely to be minor differ- 

 ences between different electrodes. To be able 

 to apply the graphic chart to all cases requires 

 that the straight line be capable of being 

 shifted, parallel to itseH at' any one temi)era- 

 ture, to correspond to the fundamental poten- 

 tial of the reference electrode being used. 



Since it is a straight line relationship with 

 which we are dealing, and since the varia- 

 tions mentioned do not change the slope of 

 line, an instrument of the slide-rule pattern 

 is not only feasible, but highly practicable. 

 For convenience, the circular type was chosen. 

 The Cnt and pn scales are engraved on a 

 disk 125 mm. in diameter. From the relation 

 between these two quantities, their main divi- 

 sions coincide ; e.g., for pn = 8, Ch+ = 10"'. 

 2 A graphic conversion chart of the kind men- 

 tioned is reproduced in "Eleetrometric Methods 

 and Apparatus for Determining Hydrogen-ion Con- 

 centrations, " L. & N. Co., 1920, p. 25. 



