115 



immersing the delivery end of the apparatus in the mercury, the 

 latter of course runs back a little way into the capillary tube, but 

 this constitutes only a very slight compression and not, in such 

 a case as this, a recognizable change of volume if the other end 

 of the apparatus has been closed at the time of this operation. 

 It will also be necessary to determine the displacement of the 

 material put within the tube, and where a considerable number 

 of similar determinations are to be made this is most easily and 

 sufficiently done by determining once for all the specific volume 

 of the objects to be experimented with. After this has been 

 done, simply weighing the tube before and after introducing the 

 material will give the necessary correction to be applied to the 

 gross volume of the apparatus. That this method gives a very 

 fairly accurate result is shown by the following analyses. Sev- 

 eral samples of the gas within the tube were taken in the manner 

 described and then the remaining gas was collected bodily by dis- 

 connecting the tube and forcing all of the gas out into a receiver. 

 The analyses were made on the Bonnier-Mangin apparatus. 



Control Experiment for Determining Accuracy of Apparatus 



A number of germinating grains of wheat in apparatus for two hours. 



Sample No. I (taken as described) per cent. C0 2 3.57 



" No. 2 " " 



it a a 



3.61 



" No. 3 " " 



tt a tt 



365 



Remainder of gas in tube, 



analysis No. I " " 



3-59 



" " " " " 



a a 2 a a 



3.55 



While this shows a little variation it is no more than is to be 

 expected, and not enough to invalidate the method for the pur- 

 poses required. It is not supposed that this method is accu- 

 rately quantitative in the sense of the chemist who is investigat- 

 ing atomic weights. It is the mistake of the novice to employ 

 methods which exceed in accuracy the necessities of the case ; in 

 dealing with living organisms the individual range of variation 

 may be, and usually is, so great that it is absurd in the majority 

 of instances to carry results out further than five in the second 

 decimal place. 



Another object in constructing this apparatus was to be able 

 to subject the plant parts under investigation to atmospheres 



