RESPIRATION 27 



Put a clean fresh shell of a river mussel, or any 

 shellfish or if there is room, several shells in a 

 pot of soil, their inner sides facing upwards, and sow 

 seeds in it. After a few weeks take the soil out and 

 examine the shells. On the shining inside surface 

 you will find very thin lines. These are made by 

 the root-hairs, the CO 2 secreted by them dissolving 

 the calcium carbonate of which the shell is composed. 



9. We have learnt that wood contains a very small 

 quantity of mineral substances, which are incombustible 

 and constitute the ash, and that these are taken up 

 out of the ground by the roots. But what about the 

 rest, the part that does burn ? 



The products of ordinary combustion are carbon 

 dioxide and water-vapour. You may prove this quite 

 easily by holding a burning match or splinter of wood 

 in the mouth of a test-tube half full of lime-water (i.e. 

 water with a little freshly roasted chunam in it) for a 

 minute or so, then shaking the tube up. A cloudy ap- 

 pearance, due to the formation of carbonate of lime or 

 chunam, shows that carbon dioxide is produced in the 

 burning. 



Now hold over a burning match a clean cold piece 

 of glass or of brightly polished metal. The bright 

 surface will be clouded for just an instant by a deposit 

 of moisture showing that water-vapour is also formed 

 and is condensed by the cold glass. The same cloudy 

 appearance can be seen whenever a cold glass chimney 

 is put on a newly lit paraffin lamp. It passes away 

 at once as the glass becomes warm. 



Carbon dioxide and water-vapour therefore are pro- 

 duced by the combustion of wood that is, by the 



