14 ACIDITY AND GAS INTERCHANGE IN CACTI. 



imenters. This valve was set so as to make the flow constant at the least 

 suction possible that would guarantee a steady stream of air through the 

 absorption tube. It is perhaps not necessary to say that, even with all these 

 precautions, the apparatus was constantly watched and in only a few instances 

 was there any trouble with the air-current. To obtain satisfactory results 

 with any suction apparatus of this sort it is essential that the flow of air or 

 gas should be as even as possible. For most of the work the flow was measured 

 by a gas-meter of the American Meter Company, which could be read to 

 0.005 cubic foot, and a rate of 5 liters per hour was used. In Tucson, in the 

 absence of the meter, the gas-volume was measured by displacement of water. 

 A by-pass was arranged in the suction apparatus so that without disturbing 

 the adjustment of the mercury valve a much faster stream could be allowed to 

 pass through the Pettenkofer apparatus when it was desired quickly to change 

 the atmosphere in the receiver. It should be added, however, that the pres- 

 sure was never allowed to become diminished by more than a few millimeters 

 of mercury, lest the relation of the internal gases in the tissues be disturbed. 



The amount of rubber tubing with which the gases from the plant came in 

 contact was cut down to a minimum, and where its use was necessary the 

 inside of the tube was vaselined so as to prevent, as far as possible, any diffusion 

 of the carbon dioxide. These rubber connectors were frequently renewed, so 

 that any deleterious effect that the vaseline might have on the rubber need not 

 be considered. 



Constancy of temperature was insured by immersing the receiver holding 

 the plants in a large container filled with water. Lacking automatic control, 

 this was kept at the desired temperature by constant watching. For raising 

 the temperature one of the familiar types of immersed electric heaters was used 

 and for lowering it cold water or, if necessary, ice. A paddle stirrer was kept 

 in agitation from time to tune to insure even distribution of heat. Where pure 

 hydrogen or nitrogen was used great care was taken to wash the gas and to 

 eliminate all traces of oxygen. A by-pass in the supply tubes made it possible 

 to sample the gas passing through the apparatus without disturbing its flow 

 for a measurable length of time. When hydrogen was used it was manufac- 

 tured in the usual way and purified twice before being run into the apparatus. 

 For the most part, however, nitrogen was preferred where an atmosphere with- 

 out oxygen was desired. The commercial product is delivered in high-pressure 

 cylinders and contains only a small percentage of oxygen and no poisonous 

 gases, as far as the writer is aware. Besides being led through the usual tubes 

 containing potassium permanganates, potassic hydroxide, etc., this was washed 

 of its oxygen by being passed through two extra long Emmerling towers and a 

 whole battery of Pettenkofer tubes filled with alkaline pyrogallol. Repeated 

 tests showed that no trace of oxygen was left after this treatment. 



As an absorbent for the carbon dioxide, the writer preferred the usual barium 

 hydroxide, except in the few instances where the apparatus was left to run 

 a long time without change, when the heavy precipitate would clog the even 

 flow of the air-current. In such cases a solution of potassic hydroxide was 

 used. The amount of carbon dioxide given off was determined as usual by 

 the decrease in alkalinity of the barium-hydroxide solution in the Pettenkofer 

 tube. In conclusion it may be said that toluene vapor was introduced into 

 the gas flowing through the apparatus, to obviate the difficulties arising from 

 the respiratory activity of possible bacterial form.?. 



