AIR AND OXYGEN 39 



the U-tube shows the weight of the compound produced. By 

 subtracting from this the weight of the sulphur taken, we get 

 the quantity of oxygen with which it has combined. 



In one experiment, the weight of 

 sulphur was 1.21 g. The weight 

 gained by the U-tube was 2.42 g. The 

 difference (= oxygen) is 1.21. The 

 proportion of sulphur to oxygen in pj G 22 



sulphur dioxide is therefore 1.21 : 1.21 



or 1 : 1 or, in percentages, 50 : 50. This proportion is very close 

 to the accepted value, 32.06 : 32. 



The same method could be used for carbon, for the carbon 

 dioxide produced would be absorbed in the solution of potassium 

 hydroxide. 



Combustion. Violent union with oxygen is called, in 

 popular language, combustion or burning. Yet, since similar 

 vigorous interactions with other gases are common, the term 

 has no scientific significance. Even the union of iron and sul- 

 phur gives out light and heat, and is quite similar in the chemical 

 point of view to combustion. 



A misleading term often used in this connection is kindling 

 temperature. It gives the impression that there is a definite tem- 

 perature at which combustion will start. But the temperature 

 is only one of the conditions which produce combustion. Finely 

 powdered iron will start burning at a lower temperature than will 

 an iron wire, because it presents relatively more surface to the gas. 

 Again, if the oxygen is at less than one atmosphere pressure, the 

 wire will require to reach a higher temperature before combustion 

 will begin. Finally, the vapor of methyl alcohol and air requires 

 to be raised above a red heat before combustion starts, but a 

 pocket cigar-lighter sets fire to this very mixture by means of a con- 

 tact agent (a thin platinum wire) without any other means of 

 heating being required. Therefore, the conditions under which 



