THE FOOD OF THE PLANT 



35 



Fig. 15 



stopper. A certain amount of air has thus been 

 enclosed in the bell-jar above the sur- 

 face of the water (Fig. 15). 



Again remove the stopper and ignite 

 the phosphorus by means of a hot 

 glass-rod or wire. Quickly replace the 

 stopper. 



Observations. — The phosphorus burns 

 brightly, producing dense fumes. The 

 level of the water mside the jar falls a 

 little at first, then rises. When the 

 burning is over, the water has risen 

 about one-fifth of the height of the jar. 

 The white fumes gradually dissolve in 

 the water. 



As soon as the remaining gas is free 

 from fumes, it is tested and the following observations 

 made. (Several jars of gas will of course be necessary.) 



1 1. It is invisible, colourless, and odourless. 

 ? 2. If soluble in water, the solubility can only be 

 slight, since the level of the water remains constant 

 when the burning is over. 



3. It is neutral to litmus-paper. 



4. A lighted taper is extinguished and the gas itself 

 does not burn. 



5. It seems slightly lighter than air. 



6. Lime-water is not turned milky by it. (Some- 

 times a slight milkiness is observed ; this will be again 

 referred to later.) 



Inferences. — 1. Air is altered »both in character and 

 quantity by the burning of a substance in it. 



2. One-fifth of the air disappears ; since this cannot 

 have escaped, it must have united with the burning 

 substance. 



3. The gas that is left, which is four-fifths of the 

 whole, is quite different in character. It can no longer 

 support combustion. 



4. Air must be made up of at least two gases, an 

 active gas that unites with phosphorus producing a white 



