120 



JUNIOR GRADE SCIENCE 



The small hole is closed by the rubber stopper. Heat the tube gently, and when 

 the phosphorus melts give a sharp flick, so as to cause the molten phosphorus 

 to run down the tube. When cool invert the tube and push it down in a 

 cylinder of water, thereby uncovering the hole in the side of the plug and 

 admitting water which rises in the tube. Raise the latter till the water in 

 it is at the same level as that in the cylinder. Measure the length of the 

 tube and of the water which has entered it. 



iii. Volume of air used up when phosphorus burns. Demonstration. 1 

 Cut a slit in a large cork so that the handle at the top of a crucible lid will 

 fit into it. Place the lid upon the cork and float on water contained in a 

 pneumatic trough. Dry most carefully with filter paper a piece of phos- 

 phorus about the size of a large pea, and place it on the floating lid. Cover 

 with a bell jar from which the stopper has been removed. Close the jar 



FIG. 29. Burning of phosphorus in a confined volume of air. 



with a cork fitted with a short right-angled glass tube, sealed at the end, 

 and containing a little sand. Note the height of the water in the bell jar. 

 Heat the end of the tube containing sand, and then give the jar a smart tap 

 on the side opposite to the tube. Hot sand falls on the phosphorus and 

 ignites it. At first the water in the jar falls. Why ? Then it slowly rises. 

 Note the height to which the water rises, and compare it with the amount 

 of air present before the burning of the phosphorus. Note that when the 

 bell jar is removed the phosphorus again takes fire. 



iv. Increase of mass when phosphorus burns. Procure a hard glass 

 tube, having the shape of BA in Fig. 93, and loosely pack asbestos fibre 

 into its drawn-out end. When this is done determine the mass of the tube 

 by weighing. Place a small piece of dry phosphorus, the mass of which is 

 about one-fifth of a gram, in the tube, and after putting it in weigh the 

 tube and its contents again, so that the exact mass of the phosphorus may 

 be known. Attach the narrow end A of the tube to an aspirator. As the 

 water runs out of the bottle a current of air is drawn through the tube, 

 and, in order to dry the air, it is first made to pass through the test-tube C, 

 containing strong sulphuric acid, to absorb the moisture from the air which 

 passes through the tube. Slightly warm the phosphorus ; it soon catches 

 fire. When this happens, remove the burner and allow the phosphorus to 



1 To be performed by the teacher. 



