July 21, 1916] 



SCIENCE 



107 



in equilibrium with the CO., of the air before 

 the tube is clamped. The closed tube is in- 

 verted several times and the color of the solu- 

 tion is compared with a series of buffer solu- 

 tions of known hydrogen ion concentration 

 and the acidity at the beginning of the experi- 

 ment is recorded. The tube can be put on a 

 shaker, should conditions require it, and after 

 any interval whatsoever, the tube is inverted 

 a few times in order to stir the liquid and to 

 get a uniform color throughout the solution 

 and then by comparing it with the buffer solu- 

 tions, the increase in hydrogen ion concentra- 

 tion is noted. This can be repeated any num- 

 ber of times and at any interval of time. 

 Changes in the hydrogen ion concentration 

 as small as from 2 X 10" 6 to 1 X 10~ 6 can be 

 detected in this way. 



Much smaller differences in the hydrogen ion 

 concentration of a solution can be detected by 

 using distilled water nearly or entirely free 

 from COy or by using solutions in which the 

 hydrogen ion concentration is low. The pro- 

 cedure when pure distilled water is used is the 

 same as that just given except that while the 

 tube is still in the bath ready for clamping, a 

 CO,-free gas is bubbled through the solution 

 until, by comparison with the buffer solution, 

 it is known that the solution in the tube is 

 between PH + 7.0 and PH + 8.0. The tube is 

 then clamped off as before and the hydrogen 

 ion concentration is read at intervals by com- 

 parison with buffer solutions. If the solu- 

 tions, due to added reagents, are quite acid, 

 then the smallest amount of C0 2 that can be 

 detected is increased. However it is often 

 possible to add the same amount of alkali to 

 each tube so as to decrease the hydrogen ion 

 concentration at the start and in this event 

 the method can become extremely sensitive so 

 as to detect minute traces of C0 2 . This is 

 also true of many solutions in which the 

 hydrogen ion concentration is very small. 



When the respiration of roots is studied, the 

 glass tube has both ends open and tubing on 

 each end. The roots are inserted, into one 

 (very short) paraffined rubber tube, and by 

 means of a pinchcock, the tube is clamped so 

 that only a small space is left about the stalk 



as it protrudes. A low melting mixture is used 

 to make the final seal about the plant. After the 

 plant has been inserted, the paraffined tube is 

 attached at the other end. The solution is then 

 run in and the C0 2 expelled by bubbling hydro- 

 gen through. The paraffin, before clamping 

 takes place, should be rather soft and pliable, 

 and should it tend to become brittle it can be 

 kept soft by being kept inside of a tube open 

 at both ends and which is kept warm by a sur- 

 rounding water bath. After clamping, read- 

 ings are made as usual. 



When the liquid used is pure distilled water, 

 and is quite free from CO,, a change in the 

 hydrogen ion concentration as small as from 

 2 X 10- 8 to 3 X 10" s can be noted. The smaller 

 the hydrogen ion concentration of the solution 

 at the start of the experiment, the more minute 

 the differences which can be detected. If the 

 experiment is started with the solution in 

 equilibrium with the CO, of the air, it is pos- 

 sible to ascertain whether or not the increased 

 acidity has been due to the giving off of C0 2 

 or to acid excretions other than C0 2 , by pour- 

 ing the solution into another tube and (after 

 shaking without the material) letting the solu- 

 tion come again into equilibrium with the air, 

 and noting whether or not the solution returns 

 to its original hydrogen ion concentration. 

 Furthermore, by bubbling a C0 2 -free gas 

 through the solution at the end of the experi- 

 ment and through a sample of the original 

 solution, it is possible to find out whether 

 acids other than carbonic acid have been 

 given off. If at the end of an experiment it is 

 found that acids other than carbonic acid 

 have been given off, or that an unequal ab- 

 sorption of ions has taken place, so as to pro- 

 duce acidity, then the increase in the hydrogen 

 ion concentration due to C0 2 can be obtained 

 by subtraction. As it is important to know 

 whether acids other than carbonic are given 

 off by plant and animal tissues, experiments 

 have been conducted upon the excretion of 

 acids by plant tissue, the results of which will 

 appear at a later time. 



When it is desirable not to have the indi- 

 cator in the solution during the experiment, 



