16 PRACTICAL AGRICULTURAL CHEMISTRY [37-40 



the wire and stopper the bell-jar. The interior will first fill 

 with white fumes, then the phosphorus will go out, and finally 

 the white fumes will disappear, leaving the interior clear 

 again. As the fumes disappear the water will rise in the 

 bell-jar, showing that the volume of gas inside has con- 

 tracted. 



37. The explanation of this is as follows : As the 

 phosphorus burns it combines with the oxygen of the air to 

 form phosphorus pentoxide, which is a white solid body, 

 and hence appears as a white cloud. When all the oxygen 

 has been used up the phosphorus goes out. Next the 

 phosphorus pentoxide dissolves in the water, and so 

 practically nothing is left in the bell-jar except the nitrogen, 

 which occupies only four-fifths of the total volume of the 

 oxygen and nitrogen originally present. 



38. Experiment. — Nitrogen neither burns nor does it 

 support combustion. To test this add water until the level 

 is the same outside as it is inside the bell-jar, open the 

 stepper and plunge the lighted taper into the gas ; the taper 

 will at once be extinguished. 



Preparation and Properties of Carbon Dioxide 



39. Fit a Woulffe's bottle with thistle-funnel and 

 delivery tube as in fig. 18. Place in the bottle a few 

 pieces of marble. Pour down the thistle-funnel enough 

 water to cover the marble, then add a little strong hydrochloric 

 acid. A brisk effervescence will take place, and carbon 

 dioxide will be evolved. 



40. Place a cylinder under the end of the delivery 

 tube. Since the gas is so much heavier than air it will 

 fall to the bottom. After the apparatus has been work- 

 ing for about half a minute hold a lighted taper just over 

 the mouth of the jar. When the jar is full the gas will come 

 in contact with the taper and extinguish it. 



