THE ATMOSPHERE 



227 



It will be easy for the student to verify this by burning in air 

 the different substances which were burnt in oxygen, and care- 

 fully comparing the products. 



Gain of Weight during Burning. If the explanation given 

 above be correct, it is evident that substances should gain 

 in weight when burnt, i.e. the products of combustion should 

 weigh more than the original substances. Do they ? We may 

 test this, for, say, phosphorus, sulphur, and magnesium wire. 



EXPT. 229. Weigh a crucible and lid and a piece of magne- 

 sium, which, folded lightly, is placed in the crucible. Heat 

 strongly in a burner, taking care to let no fumes escape. To 

 do this, keep on the lid, and only raise a little when the flame 

 is removed. The magnesium is seen to burn brightly in 

 places ; but, if care is taken, no fumes are lost. When 

 finished, the whole mass should be in the form of a white 



tosuuctionpump 



'Phosphorus 



'Asbestos fibre 



sulphuric 

 tfcidU 



FIG. 114. To illustrate Experiment 230. 



powder. Allow to cool, and weigh the crucible with the lid 

 and powder. Subtract the weight of the crucible and lid to 

 find the weight of the powder. Is it more or less than the 

 weight of magnesium ? It will almost certainly be found to 

 be more, and, if the experiment is carefully done, it will be 

 found that the magnesium has gained about 66 '5 per cent. 



EXPT. 230. Get a hard glass tube, BA in figure, pack the 

 drawn-out end with asbestos fibre ; weigh carefully. Place 

 in it a small piece of dry phosphorus (about *2 gram) and 

 again weigh. Connect up as shown in figure, the end A being 

 attached to a suction-pump which can be fitted to a tap. This 



Q 2 



