EXERCISE V. 

 NITROGEN— AN ELEMENT OF PLANT FOOD. 



Supplies for a Laboratory Section of Twelve. An ounce of pyrogallic acid; strong potassium hydroxide solu- 

 tion; twelve rulers; two tanks or basins filled with water; twelve test tubes. 



1. Put enough pyrogallic acid into a test tube to fill the rounded part. Add water until 

 the tube is filled to a depth of a half inch. To this add an equal amount, or a little less, of 

 potassium hydroxide solution. Close the tube tightly with the thumb. The upper part of the 

 tube contains what? Measure with a ruler and state its depth in centimeters. 



2. Shake the tube thoroughly for a short time, being careful to let no more air enter; then 

 hold it upside down with the mouth below the surface of the water in a tank provided for 

 this purpose, (a) Observe any change which takes place within the tube- when the thumb is 

 removed. ''Before removing the tube from the water again place the thumb over the mouth, 

 that the water which has entered may not escape. The cause of the action observed is that the 

 liquid mixture in the test tube absorbs the oxygen, leaving almost pure nitrogen in the space 

 which was before filled with both nitrogen and oxygen. The vacancy thus made is filled in 

 by the water from the tank, which is pressed in by the pressure of the outside atmosphere, 

 (b) Observe the color of the nitrogen in the tube, (c) What common observation also tells 

 you that nitrogen is a colorless gas? 



3. (a) Measure and state the depth of the nitrogen in the tube, (b) This depth of nitro- 

 gen is what part of the depth of the air which you measured in experiment 1? (c) From your 

 measurement, calculate what fraction of the air is nitrogen, (d) Infer what portion must be 

 oxygen. 



4. Lower a burning match into nitrogen, (a) Observe what occurs. Try to light the 

 gas. (b) Observe whether or not it will burn. 



5. Make a brief summary of the important points studied as suggested by the following: 

 (a) Does nitrogen have color? (b) What portion of the air is nitrogen? (c) What portion 

 of the air is oxygen? (d) Will nitrogen burn? (e) Will nitrogen allow other substances to 

 burn in it? (f) Why is nitrogen, as an element of plant food, more expensive than oxygen? 

 Reference. "Elements of Agriculture," Warren, page 116. 



•Note: Though the dark brown liquid diffuses outward in the basin of water, it does not necessarily follow 

 that the original volume of the liquid in the test tube has been decreased. 



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