SEEDS AND SEEDLINGS. 2 



enough, the presence of sugar in the water outside of the thistle-funnel 

 may be detected by tasting. 



(&) Repeat the experiment, using common salt (about 20 per cent. ) 

 instead of sugar in the bulb and distilled water instead of tap water 

 in the jar. From time to time take a little water out of the jar, put it 

 in a test-tube, and add a drop of silver nitrate ; a white precipitate 

 shows the presence of salt in the water. 



When the liquid stops rising in the tube take the thistle-funnel out, 

 replace the water in the jar with a very strong solution of salt, put the 

 bulb in again, and note that the liquid now falls in the tube (why ?). 



53. Osmotic Pressure. Careful experiments with suitable mem- 

 branes have shown that the osmotic pressure set up by a 1 per cent, 

 solution of saltpetre is enough to support a column of water over 

 100 feet high, i.e. equal to a pressure of 3| atmospheres, or more than 

 50 Ibs. per square inch. Some idea of osmotic pressure may be ob- 

 tained by using apparatus fitted up like that shown in Fig. 16B (a 

 thistle-tube like this costs about 6d. ). The pressure can be measured 

 by pouring mercury into the bent tube, so that at first the mercury is 

 at the same level in the two limbs of the tube, and noting the extent 

 to which the mercury is pushed down in one limb and rises in the 

 other. The cork should be split so as to fix the tube in it (this is not 

 shown in Fig. 16A, but the use of the cork is merely to hold the tube 

 in position). 



The pressure set up by endosmosis has an important application 

 in connection with root-absorption and other processes in plant-life. 



54. Properties of Water. Since water is absolutely 

 essential in connexion with the germination of seeds, and 

 other processes in plant life, you should study, in books on 

 Chemistry and Physics, the various chemical and physical 

 properties of water. Get clear ideas on the following topics, 

 and as far as possible make experiments for yourself, unless 

 you have already gained a thorough practical knowledge of 

 the subject. 



The three states in which water exists, the impurities of natural 

 water ; filtration, solution, evaporation, distillation, condensation ; 

 the gases dissolved in natural water ; the thermometer ; alcohol, mer- 

 cury, maximum and minimum, and black-bulb thermometers ; change 

 of state ; freezing and boiling points ; expansion of water when heated 

 from 4 C. to 100 C., and its expansion when cooled from 4 C. (the 

 temperature of maximum density of water) to C. ; conduction and 

 convection of heat ; specific heat ; latent heat ; unit of heat ; com- 

 position of water, as shown by analysis and by synthesis ; properties 

 of hydrogen and of oxygen. 



