and Laboratory Methods. 



2175 



Pass in 8 or 10 c. c. of pure carbon dioxid, which has been freed from hydro- 

 chloric acid by washing in sodium bicarbonate solution, then note the volume 

 accurately. The apparatus is now exposed to bright diffused light for six to 

 twenty-four hours. When the exposure 

 is completed the leaf must be pulled out 

 by means of the wire. After the appa- 

 ratus has returned to room temperature, 

 readings are again taken. By means of 

 a curved pipette introduce 1 or 2 c. c. 

 of a concentrated solution of potassium 

 hydroxid into the tube, using great care 

 not to admit any air. At the end of two 

 hours the surplus of carbon dioxid will 

 be absorbed, and careful readings are 

 again taken. The volume of the leaf 

 must be ascertained and subtracted from 

 the gas volume ; it may be found by 

 immersing the leaf in 50 per cent, alco- 

 hol, contained in a graduated cylinder. 

 To obtain the absolute volume, it is ne- 

 cessary to reduce the readings (before 

 and after the injection of the potassium 

 hydrozid) to 0°C. and 1 meter mercury 

 pressure and to correct for the tension 

 of water-vapor, etc. This is to be done 

 by the formula of Bunsen, 



V, 



.(v-m) (b-bj-b^) 



l + 0.00366t° 

 where Vj ^reduced volume of the gas, 

 v^the observed volume, 

 m^the correction for the 



meniscus, 

 b=the barometric reading, 

 b^^the mercury pressure in 



the tube, 

 b2=the water-vapor tension at 

 the temperature t°. 

 ((■) Engehnanii''s Bacterial Method. 

 — In this method the presence of oxygen 

 is detected by the extreme sensitiveness Fig. 8. 



of certain bacteria to that gas. It is 



claimed that the presence of the billionth part of a milligram of oxygen can be 

 ascertained by this test. 



In order to obtain the bacteria, kill a pea by placing it in boiling water, then 

 allowing it to putrefy in 200 c. c. of water. The best results will be obtained if 

 the bacteria thus procured are transferred to agar cultures. 



