THE FOOD OF PLANTS. 61 



mercury. It is often advisable also not to place the swollen seeds 

 at once on the covering of the beaker, bat first let them germinate 

 under the bell-glass on moist wool. The tubulnre of the bell-glass 

 is provided with a 2-holed rubber stopper, through which pass 

 the two tubes a and &, both bent at right angles. Through a 

 is led atmospheric air, which however has first been freed from all 

 nitrogenous compounds, and again saturated to some extent with 

 aqueous vapour. For this purpose the air before it passes into a, 

 is led through a series of wash bottles, of which the first contains 

 a solution of Sodium bicarbonate, the second pumice stone soaked 

 with Sulphuric acid, and the third water. The tube 6 is con- 

 nected up with an aspirator, but it is well to interpose between 

 this and fe another small bottle containing Sulphuric acid, so that 

 the air under the bell-glass is not in direct communication with 

 the atmosphere at all. (Respecting the method of using the 

 aspirator, see the section on Respiration.) 



If a continuous slow stream of air is led through the apparatus 

 from the time the seeds are set to soak, the plants being exposed 

 to bright diffuse light, they develop as well as is possible in ab- 

 sence of Nitric acid and ammonia. The experiments are carried 

 on for, say, fourteen days or even longer, air being passed through 

 the apparatus day and night during the whole time. It is then 

 necessary to determine the quantity of Nitrogen contained in the 

 seedlings produced. The plants are reduced to a pulp in a porce- 

 lain dish. The weight of the dish, and also that of a glass rod 

 used for stirring the pulp must be determined. We then place 

 the dish on a water bath and add to it the balance of the food 

 solution, evaporated to a small bulk, in which the roots of the 

 plants developed, together with the residue from the soaking 

 water and from the water used for washing out the glass wool on 

 which the seeds may have been germinated. When the mass in 

 the dish has become fairly dry, it is placed for some time in a 

 drying chamber at a temperature of about 50 C. The dish, 

 loosely covered, is then exposed to the air for twenty-four hours. 

 We ascertain the weight of its air-dry contents, and then at once 

 determine the corresponding dry weight. Nitrogen determina- 

 tions are also to be made.* If we now finally compare the 



* In estimating the Nitrogen in the whole of the research material, it is speci- 

 ally desirable to dry the macerated seedlings, together with the residues from 

 the soaking water and food solution, in Hoffmeister dishes (to be obtained of 

 Muencke, in Berlin). 



