Exchange and Movements of Gases and Liquids. 81 



the air in these spaces, especially with regard to the propor- 

 tion of oxygen and carbon dioxide present. 



Smooth both ends of a glass tube about 30 to 60 cm. long, 

 fit a cork stopper over one end, and place its upper surface 

 nearly flush with the end of the tube. Coat the end of the 

 cork and glass tube with soft sealing wax. Cut a small piece 

 of coarse, strong cloth, or, better, fine brass gauze, and imbed 

 it in the warmed wax. Now lay a thin slice from the outer rind 

 of a squash, the cuticle of an apple, or skin of a grape, on the 

 gauze or cloth, and apply more wax in such manner that the fit- 

 ting is made perfectly tight, and no gas may pass except through 

 the tissue. Care must be exercised to select material free from 

 perforations and faults. Lay the tube nearly horizontal, and, 

 after the wax is cool, run in distilled water until it is com- 

 pletely full and all air is expelled. Stand upright in a dish of 

 mercury for an hour. If the fitting is properly done, the water 

 will be held in the tube, and no air will enter through the mem- 

 brane. Now displace the water with carbon dioxide or oxygen 

 taken from a receiver or generator (Figs. 17 and 18). Note 

 the height of the mercury column daily, as the gas diffuses out 

 through the membrane. 



