128 COMBUSTION OF MAGNESIUM. 



Doint, as to combine with them with great rapidity and 

 violence. In doing this they develop or expend so much 

 force as to produce a great quantity of light and heat, 

 which are considered as only two of the many forms of 

 force. To commence this process of rapid combination 

 with oxygen, a portion of the substance must first be heat- 

 ed to the requisite point ; but, when it is once commenced., 

 it goes on, the heat developed by the combustion raising 

 the successive portions to the right temperature for con- 

 tinuing the process. This heating a portion of the com- 

 bustible in order to commence the process is what we call 

 kindling the fire. All this has already been explained, and 

 must not be forgotten. 



Now, when substances are burned that is, are delivered 

 over to this eager and fierce seizure of their particles by 

 oxygen, the compounds that are produced are called the 

 products of combustion, and these products, of course, vary 

 very much according to the nature of the substances com- 

 bined. Sometimes they are gases which rise into the air. 

 Sometimes they are powdered solids. In the case of mag- 

 nesium, the product is the well-known white powder mag- 

 nesia, which is, in chemical language, the oxide of magne- 

 sium, or, as it now is sometimes proposed to call it, magne- 

 sium oxide. 



John knew all this, so that when Lawrence told him 

 there Avould be no danger from fumes in burning his mag- 

 nesium, he was ready to assent to it at once. 



"But, then," said Lawrence, "there is sometimes a possi- 

 bility that some fused portion of the substance to be burned 

 may fall down, and do harm in that way. This happens 

 when the heat produced melts the substance faster than 

 there is oxygen at hand to combine with it. I do not know 

 how this may be with magnesium, and so, in order to make 

 our experiment perfectly safe, we will ask the landlady to 



