90 



LABORATORY EXERCISES 



cup or small glass, small potted plant (see e), glass tube, rubber tube or 

 adhesive tape. 



a. In this exercise we are to study the phenomenon called 

 osmosis, or the passing (diffusion) of water and dilute solutions 

 through thin membranes. In this way the dilute soil solution 

 enters the root hairs of the plant. We can study osmosis most 

 easily by the use of an egg (Fig. 32). 



Tap the large end of an egg so as to crack the 

 shell; then pick off the shell, bit by bit, from an 

 area about as large as a cent. Be very careful 

 not to break the thin membrane ("skin") inside 

 the shell. Crack the small end also, but do 

 not remove the shell from an area more than 

 1 or 2 mm. in diameter. Stand the egg in a 

 napkin ring, or in the mouth of a bottle, so that 

 the smaller end is upright. 



b. Select a piece of glass tubing about 7 cm. 

 (3 in.) long. From one end of a candle cut off, 

 by means of a hot piece of iron wire (a thick hair 

 pin will do), a round piece about 6 mm. (% of 

 an inch) thick. Take out the wick, and with 

 the heated wire enlarge the wick hole so that 

 the glass tubing can just be pushed in. Use the iron wire 

 (reheat it when necessary) as a "soldering iron" to fasten the 

 edge of the wax securely to the glass. See that the opening 

 of the glass tube is not closed with wax. 



Now hold the glass tube so that the piece of wax is against 

 the small end of the egg, and so that the hole in the glass tube 

 is just over the opening in the shell. With the heated wire 

 melt some of the wax, and fasten the lower edge of the wax 

 securely against the egg shell. 



c. Push a hat pin through the glass tube, and thus break the 

 skin in the small end of the egg. Put the egg in an egg cup or 



FIG. 32 



