86 ACIDITY AND GAS INTERCHANGE IN CACTI. 



In all of the three experiments, with a not especially high acidity, there is 

 a high ratio, which averages close to unity; while the corresponding control 

 experiments in ordinary air show a ratio that is nearer 0.75. The increase of 

 carbon dioxide is shown by these experiments to be from twice to three times 

 that which it is in air. In the acidity experiments already described, it has 

 been shown that there is a diminution of acidity with increased oxygen, so 

 that it seems probable that the effect of the additional supply is largely to 

 hasten the disintegration of the acids. High gas ratios can only mean an 

 increase in the breaking-down processes which liberate carbon dioxide, unless 

 they are caused by a diminution of the amount of oxygen absorbed, which is 

 not the case in this instance. The number of experiments with increased 

 oxygen-supply was limited and none was tried with material either of very 

 high or of low acidity. In the former case we may surmise that the only effect 

 would be that the amount of carbon dioxide produced would be still greater and 

 the ratio even higher. In the latter case the effect of the additional oxygen 

 would be to prevent the accumulation of acid, and as a consequence the ratio 

 also would be higher than under normal conditions. 



GAS INTERCHANGE OVER LONG PERIODS. 



During the first summer when the gas-interchange determinations were 

 made, a considerable number of experiments were carried out in which the 

 carbon dioxide was allowed to accumulate in the receiver for considerable 

 periods. As the amount of material used in these determinations was the 

 same as that employed where the time period was much shorter, the amount 

 of this gas which gathered was considerable, amounting usually to 12 and in 

 one case to as much as 18 per cent. These have not been included in the gen- 

 eral consideration of the results, since it is known that the presence of so much 

 carbon dioxide in the surrounding air affects the ratio of gas interchange. 

 A few of these results are given in table 67. The chief effect seems to be the 

 proportional diminution in the amount of oxygen absorbed, in consequence of 

 which the ratio is high, considering the acidity of the specimen. The amount 

 of carbon dioxide is also less than under more normal conditions, which per- 

 haps may be ascribed to the effect of the carbon dioxide which accumulated, 

 rather than to the lessened oxygen supply. The long-period experiments tried 

 in 1913, where the amount of material was adjusted to the duration of the 

 experiment and no great accumulation of carbon dioxide took place, gave 

 normal results (experiments 35, 36, 29, 256, 266, 276, table 59). The occasion 

 for mentioning the experiments described above is chiefly to call attention to 

 the effect of increased carbon dioxide content of the surrounding air. They 

 have otherwise no particular significance. 



