I oo THE NA TURE- S TUB V RE VIE W [3 : 4-apr., 1907 



of the wood burned in the air carbonic acid gas — a compound gas 

 consisting of carbon and oxygen — is produced, that the wood suppUed 

 the carbon which entered into this gas, but that the oxygen must 

 have come out of the surrounding air just as it came from the sur- 

 rounding oxygen when we burned carbon in a bottle of oxygen; and 

 since the burning carbon got oxygen out of the air, the air must con- 

 tain oxygen. And so it will appear that when carbon is burning in the 

 air it is undergoing oxidation. Attention should be called to the fact 

 that this union not only produces an oxide which may be caught in a 

 bottle, but that it is accompanied by heat which may be felt, and by 

 light '^\\\q\\. may be seen. 



Next, try to burn a piece of glowing charcoal in a bottle of air. It 

 will not glow and burn brightly as it did in a bottle of pure oxygen, 

 but soon becomes cold and black again. The children will conclude 

 that the air must contain, besides oxygen, some other gas in which 

 charcoal will not burn. They may be told that this gas which prevents 

 substances from burning as well in the air as in pure, or nearly pure 

 oxygen, is called nitrogen. 



Let the pupils take a little lime-water to their homes with the re- 

 quest that they will try to find by catching in a bottle the gas from a 

 coal or wood fire whether carbonic acid gas is produced by the fire 

 in the stove. Their results should be discussed at school and the 

 questions proposed: Why we burn wood and coal in stoves — whether 

 for the light, the heat, or the carbonic acid gas produced.'* Whither 

 does this gas go and how ? — how does the wood or coal obtain oxygen 

 in a closed stove ?— and why does the burning become slower when 

 we close the draught 1 



On the evening before the next lesson the children may be asked 

 to try to char (not burn) small pieces of bread, potatoes, sugar, meat 

 and other food stuffs, on'a hot stove, and to eat for breakfast some 

 of the articles of food in which they find carbon. 



At school the problem will be to find whether the carbon in these 

 foods is oxidized in the body as the carbon of wood and coal are in 

 the stove. Now, if the carbon of the food unites with oxygen in the 

 body, carbonic acid gas should be produced and may be caught in a 

 bottle. The enquiry may be made more dramatic if the teacher will 

 get a pupil before the class to blow his breath, by means of at 

 tube, through a small bottle of lime-water. When the lime-water 

 whitens, the children will see that the carbon of the food taken into 

 the stomach appears in the breath from the lungs as an oxide of car- 



