44 INFLUENCE OF CHEMICALS 



until all of thd air in the retort has passed off. The decomposition 

 of the potassium bicarbonate takes place in a manner shown by 

 the following reaction : 2KHCO3 = K 2 C0 3 + H 2 + C0 2 . Turn 

 the cock and allow the gas to displace three-fourths of the water 

 in the gasometer. Care must be taken in the heating and the 

 cleaning of the retort that it shall not be broken. Liquid carbon 

 dioxide may be procured in steel cylinders from manufacturers 

 which will replace this material. It should be tested for im- 

 purities, and allowed to escape from the cylinder into the gaso- 

 meter at the proper pressure. After the gasometer has been 

 filled, connect its outlet tube with an Engelmann gas chamber 

 on the stage of a microscope. Two or more microscopes may 

 be placed in series and the same stream of gas allowed to flow 

 through all of the chambers. 



Now test the material examined in the last experiment. The 

 streaming movement in Tradescantia may be stopped in 1.5 to 2.5 

 minutes after exposure to the pure gas. Quickly replace the gas 

 with pure air from a small bulb attached to the inlet tube of the 

 Engelmann chambers. Motion is resumed in less than a minute. 

 Great variability in this reaction time will be found, however, in 

 different kinds of material. 



Procure a second gasometer or aspirator bottle and make the 

 following mixtures of oxygen and carbon dioxide : 



O 25 C0 2 75 



" 20 " 80 



" 10 " 90 



" 5 " 95 



Determine what strength of carbon dioxide is necessary to stop 

 movement. Select a few hairs of Tradescantia in which the move- 

 ment is very vigorous and expose them to a mixture containing 

 75 per cent, of the carbon dioxide for an hour. Then replace 

 with a mixture containing 8,0 per cent. Continue until the cells 

 are exposed to the pure gas. In many instances the exposure 

 to mixtures of successive degrees of concentration will allow the 



