ACIDITY DETERMINATIONS. 33 



bered that the New York temperatures were not as high as those at Tucson, 

 but still it is evident that in the study of these plants it is highly necessary 

 to check all results with those from work carried on in the natural habitat of 

 the plant. 



In the Tucson experiments (table 18) exposure to 75 to 80 per cent oxygen 

 was followed by a more than 50 per cent decline in acidity, showing very 

 plainly that under fairly constant conditions of temperature and in the dark 

 the oxidation of the acids will proceed more rapidly when an excess of oxygen 

 is supplied. In ordinary air, under the same conditions, the diminution of 

 acidity was considerably less, though still well marked, but in pure hydrogen 

 it was maintained at almost its original point. As in these specimens the 

 initial acidity was high probably near its maximum it would hardly be 

 expected that actual rise of acid-content would be shown in the hydrogen 

 atmosphere, even if such a phenomenon commonly takes place. It may be 

 noted, however, that one experiment does show such a slight increase. The 

 actual figures averaged from this set of experiments are as follows: for mature 

 joints with an initial acidity of the juice of 1.32 per cubic centimeter, exposure 

 for from 22 to 30 hours in air at about 30 C. gave an acidity of 0.96; in oxygen 

 an acidity of 0.60, and in hydrogen one of 1.18. Young joints under prac- 

 tically the same conditions showed the following changes from the initial juice 

 acidity of 1.84 per cubic centimeter: to 1.17 in air, to 0.66 in excess oxygen, 

 and to 1.75 in pure hydrogen. 



From the next group of experiments (table 19), made on material of low 

 initial acidity, it may be seen how the presence or absence of oxygen affects the 

 accumulation of acid in the tissues. The average acidity of the pure juice at 

 the beginning of the experiment was for the mature joints 0.23 per cubic centi- 

 meter and for the young ones 0.48 per cubic centimeter. This rose under the 

 conditions of the experiment to 0.93 and 1.65 respectively, when the material 

 was exposed to normal air. In hydrogen atmosphere the rise for the same 

 length of time is to 1.07 for old and 1.57 for young. In excess of oxygen 

 amounting to 75 to 80 per cent at the end of the experiment, the acidity of the 

 mature joints had risen slightly, being 0.37, but in the young material it was 

 practically the same as at the beginning, being 0.46. It is to be observed that 

 in one of the three experiments the acidity rose perceptibly, but in the other two 

 it fell. The average initial acidity indicated for the mature joints is exceed- 

 ingly low, and the apparent rise in the oxygen atmosphere is probably no more 

 than that represented by individual differences of the material used; at all 

 events, in neither the young nor the mature specimens is the change of acidity 

 of significant magnitude. 



In an excess of oxygen, then, the acidity does not rise in material of low 

 initial acidity; it shows no decrease, however, presumably because the mini- 

 mum has already been attained. In hydrogen, on the other hand, there is an 

 accumulation of acid of essentially the same amount as in normal air. This 

 last result, as shown by this series of experiments, is not, indeed, in agreement 

 with what others, particularly Astruc, have found with different succulents. 

 It was, however, no surprise to have found such a marked increment in acidity 

 in the absence of oxygen. Of course, at the beginning of the experiment, as 

 has been explained in other cases, there remained a small percentage of oxygen, 

 but not enough to account for the appearance of so large a quantity of acid 



