122 JUNIOR GRADE SCIENCE 



What happens when phosphorus burns in this way ? Is the change 

 anything like that when tin calcines ? Does the phosphorus gain or 

 lose in mass ? These and several other questions present themselves, 

 and they must be answered in this lesson. 



Change produced in air by burning phosphorus. To decide 

 whether phosphorus in burning causes the same change in air as tin 

 does when calcining, it is best to burn some phosphorus in an enclosed 

 amount of air. One way to do this is to place a little phosphorus on 

 a cork or basin which floats on the surface of water, under a bell jar, 

 or a stoppered bottle having no bottom. After the experiment is over 

 and the fumes have disappeared, the water is seen to have risen in the 

 jar, indicating that there is less gas in the jar than before the phosphorus 

 was burnt in it. 



From what has been learnt already it can be understood at once 

 that phosphorus in burning takes out the active part of the air and 

 leaves the inactive part behind. So far, then, the changes which occur 

 when phosphorus burns are very like those when tin is calcined. Some 

 differences will be studied a little later. 



The fraction of the air which disappears as a result of the burning 

 of the phosphorus in a stoppered jar, can be measured easily enough 

 after the jar has been raised a little, so that its mouth is still under 

 water, but it no longer rests on the bottom of the basin. One- fifth of 

 the air is taken out of it by the phosphorus in burning. 



Another way is to burn a little phosphorus in a closed tube, then, 

 by opening the tube under water, the volume of water which takes the 

 place of the air used can be found, and its proportion to the whole 

 volume of the tube can be determined. 



That the gas left behind is really the inactive part, which is called 

 nitrogen, can be proved by quickly pulling out the stopper of a jar 

 in which phosphorus has been burnt, and introducing a lighted taper. 

 The flame is at once extinguished. 



Phosphorus slowly takes out the active part of the air without 

 being lighted. When a piece of clean phosphorus is exposed to an 

 enclosed quantity of air over water, the rapid changes described in the 

 last paragraph take place slowty. The only difference in the two cases 

 is the rate at which the active part of the air is taken out. Burning 

 phosphorus combines with the active part very quickly ; ordinary 

 phosphorus but slowly. Still, given time enough, ordinary phosphorus 

 will remove all the active part of air, and at the end of the experiment 

 it will be found that again one-fifth of the air has disappeared. 



Gain of mass when phosphorus burns. If a piece of phosphorus 

 is put into a closed vessel of air, and the whole is weighed before and 



