CONSTITUTION OF AIR 123 



after the phosphorus has been burnt, one would expect the mass to be 

 the same in both cases. To begin with, there would be the flask, 

 the air in it and the phosphorus ; after the phosphorus had burnt 

 there would be the flask, the white powder and the inactive part 

 of air. As phosphorus uses up the active part of air in burning, and 

 as, after burning, the flask only contains the white powder and the 

 inactive part of air, evidently the white powder weighs more than the 

 phosphorus (see 65 i.). 



To determine exactly the increase in mass when phosphorus burns, 

 a piece of phosphorus is weighed and placed in a tube through which 

 a current of dry air is caused to pass. The phosphorus is set alight by 

 warming it, and the white fumes are prevented from passing out of the 

 tube by some asbestos fibre placed in it. If the mass of the tube and 

 asbestos fibre is determined to begin with, and then another weighing 

 is made when the phosphorus has been consumed, it is found that 

 the tube with the asbestos fibre increases in mass, and that the white 

 powder which causes the increase weighs more than the piece of phos- 

 phorus used. If the mass of the phosphorus to begin with is one-fifth 

 of a gram, the mass of the powder produced would be nearly one- half 

 a gram. 



Properties of the substance formed when phosphorus combines 

 with the active part of the air. It has as yet only been noticed that 

 the substance which is formed when phosphorus unites or joins with the 

 active part of the air is a white snow-like powder ; this corresponds 

 to the calx formed when tin is heated in air. With a little care the 

 quick disappearance of the white material which forms the fumes of 

 burning phosphorus can be prevented. All that need be done is to 

 burn a piece of dry phosphorus in a dry vessel. In these circumstances 

 the white fumes settle down on the inside of the vessel in the form of 

 a snow-like solid. But the white powder has so great an attraction 

 for water that as soon as the vessel is opened it extracts the moisture 

 from the air, and, first becoming moist, is quickly replaced by drops 

 of liquid. If water is put into a test-tube or other vessel in which dry 

 phosphorus has been burned, the white powder rapidly dissolves with 

 a hissing noise, like that noticed when water comes in contact with 

 hot iron. When a piece of blue litmus paper is dipped into a solution 

 of the white powder produced by burning phosphorus it is immediately 

 reddened. 



