68 THE FOOD OF PLANTS 



Experiment 21. The directions for drying, heating, 

 and weighing already given apply to this experiment. 

 The weight of ash obtained is, however, very small, and 

 care must be taken to weigh very accurately. The ash 

 should be kept, as it is required in Experiment 22. 



Experiment 22. A very fine platinum wire should 

 be used here. It should be heated in the flame for some 

 time till it ceases to colour it, then moistened in a few 

 drops of hydrochloric acid in a watch-glass, then dipped 

 in the ash, and then placed in the flame again. It should 

 be held in the outer part of the flame. A pale violet tint 

 will be imparted to the flame, mixed with a yellow tint, 

 due to the presence of soda. With a little practice, 

 however, the potash violet flame will be easily recog- 

 nised. 



Experiment 23. About as much of the ash as 

 would cover a sixpence will do for this experiment. It 

 should ba emptied into a test-tube, about half an inch 

 (measured on the side of the test-tube) of strong nitric 

 acid added, and the test-tube warmed gently over the 

 flame. Then add an inch of distilled water and filter 

 into another test-tube. Add to 1 inch of the filtrate 

 1 inch of strong nitric acid, and 2 inches of molybdate 

 of ammonia solution; shake up and warm gently, and 

 allow it to stand for a few minutes. The yellow pre- 

 cipitate will be slowly formed. 



Experiment 25. The end of the tube of the 

 thistle funnel must of course dip below the water in 

 the bottle so as to prevent the gas escaping up it. Fresh 

 strong hydrochloric acid can be poured down the funnel 

 from time to time to keep up the supply of gas. The 

 marble should be broken up into pieces about the size of 

 filberts. The gas set free is carbonic acid gas. It 

 entirely prevents the charcoal from burning. 



Experiment 28. This experiment and the two 

 preceding it are due to Priestley, who first proved the 

 exchange of carbon (charcoal) between the animal and 



