THE PLAJ^T. 21 



electrical fluid over the charcoal it may be ignited, 

 when it will burn with great brilliancy. In burning 

 it unites with the oxygen forming carbonic acid, and 

 disappears. It is no more lost, however, than is the 

 carbon of wood which is burned in a stove ; al- 

 though invisible, it is still in the tube, and may be 

 detected by careful weighing, A more satisfactory 

 proof of its presence may be obtained by decornj^os- 

 ing the carbonic acid by drawing the wires a short 

 distance apart, and giving a sparh of electricity. 

 This immediately separates the oxygen from the car- 

 bon, which forms a dense black smoke in the tube. 

 By pushing tlie corks together we may obtain a 

 wafer of charcoal of the same weight as the piece 

 introduced. In this experiment we have changed 

 carbon from its solid form to an invisible gas and 

 back again to a solid, thus fully representing the 

 continual changes of this substance in the destruc- 

 tion of organic matter and the growth of plants. 



CHAPTEE III. 



UTDKOGEN, OXYGEN AND NITROGEN. 

 HYDROGEN AND OXYGEN. 



Let us now consider the three gases, hydrogen, oxygen^ 

 and nitrogen, which constitute the remainder of the 

 atmospheric part of plants. 



