130 COMBUSTION OF MAGNESIUM. 



posed of the magnesium itself, combined with the oxy- 

 gen." 



" Yes," said John, " I know. It is magnesia." 



"It is well enough to call it the ashes in common par- 

 lance," continued Lawrence, " on account of its resemblance 

 to the ashes of wood or paper in its apparent origin and in 

 its form, if we only know that it is formed, chemically, in 

 quite a different way." 



To have been perfectly precise in his statement, Law- 

 rence might have added that the ash left in the burning of 

 wood is mostly composed of compounds of certain metals 

 with oxygen, formed by some previous process analogous 

 to combustion, and left in the ground, whence they were 

 taken up by the rootlets of the plants, and built, so to 

 speak, into the wood. But the combustion, if it really was 

 a process of combustion, by which they were originally 

 produced, was not the burning of the wood, but took place 

 long before. In the combustion of the wood they were 

 simply passed over and left, whereas, in case of magnesium, 

 the magnesia which results is produced at the time, and by 

 the very process of the burning. 



"It did not drop upon the plate after all," said John, 

 looking at the plate, which remained perfectly clean after 

 the experiment. 



"No," replied Lawrence; "I was almost sure that it 

 would not. I w r as very confident that the burning would 

 keep well in advance of any tendency to melting ; but, in 

 trying philosophical experiments in a parlor, it is always 

 best to take measures for guarding against even the most 

 improbable contingencies." 



