40 



PHYSIOLOGY OF NUTRITION. 



accurate quantitative researches concerning the amount of Carbon 

 dioxide contained in atmospheric air, the volume of air passed 

 through the apparatus must be measured by means of a gas- 

 meter. (For full instructions regarding the determination of the 

 amount of Carbon dioxide in gas mixtures, see the third Section 



under Respiration.) 



Water is able to ab- 

 sorb a not inconsider- 

 able quantity of air. 

 If a glass vessel about 

 half filled with spring 

 water is placed under 

 the receiver of an air- 

 pump, the air dissolved 

 in the water at once 

 escapes on working the 

 pump, because the 

 pressure of the air 

 over the water is 

 rapidly reduced. If to 

 some spring water is 

 added clear lime water 

 or baryta water, a more 

 or less considerable 

 clouding of the fluid 

 takes place, owing to 

 the precipitation of 

 Calcium carbonate or 

 Barium carbonate as 

 the case may be. This 

 turbidity however does 

 not conclusively prove 

 that the water contains 

 free Carbon dioxide in 

 solution ; it may be 

 caused by Carbon dioxide present in loose combination in Calcium 

 bicarbonate dissolved in the water. 



Unless Carbon dioxide is present in the medium surrounding 

 plants (air or water), no assimilation is possible, no formation of 

 organic material, and naturally also no liberation of Oxygen. 

 Proof of the fact that fresh organic material can be produced in 



FIG. 15. Apparatus for proving the fact that green 

 plants can only produce Oxygen when Carbon Dioxide 

 is at their disposal. 



