RICHARDS. — CONCERNING GAS-ANALYSIS. 277 



100 X 168.9 mm. 



809.7 mm. 



= 20.86. 



The mean of all these analyses indicates 20.85 per cent by volume of 

 oxygen in the air of the laboratory, a value which is probably very near 

 the truth.* 



It is interesting to note that the difference between the results 20.94 

 per cent and 20.80 per cent is caused by a difference of reading in the 

 water column of 15 mm. Hence it is clear that a meter-stick, or a rod 

 provided with labels whose distance apart may be measured at leisure, 

 is quite sufficiently accurate as a means of measurement for ordinary 

 purposes. 



Attention should be called to several possible errors which must be 

 guarded against in this process. 



(1) Either no air should be permitted to remain permanently in the 

 tube connecting the pipette and the measuring bulb, or else the volume 

 of the residual bubble should be suitably corrected, according to circum- 

 stances. 



(2) None of the absorbing liquid should be run into the measuring 

 bulb; or if by accident some of the absorbing liquid finds its way into 

 this bulb, it should be washed away with pure water, in order that it may 

 not affect the tension of the aqueous vapor. 



(3) Care should be taken to allow time for the equalization of the 

 temperature change caused by the compression and expansion of the gas 

 as well as for the running down of adhering water. 



(4) The temperature must be kept constant, within 0.05° centigrade, 

 a condition which is easily fulfilled by stirring warmer or cooler water into 

 the open receptacle. If an accurate thermometer is not at hand, a sensi- 

 tive air thermometer may be improvised for this purpose, — for change 

 of temperature must especially be guarded against. The ingenious device 

 suggested by Professor Hempel for this correction serves well.f 



Of course all these precautions apply equally to the usual measurement 

 under constant pressure, but they are not always heeded. 



Attention may be called also to the fact that neither the constant- 

 volume nor the constant-pressure method necessarily affords the true 

 measure of the volume of the absorbed gas. This would only be true if 

 all gases were perfect gases ; as a matter of fact, no accurate measure- 



* The metliod was tried also by a large class of beginners in gas-analysis with 

 successful results. 



■1 Ileiiipel (translated by Dennis) (1902), page 84. 



