36 



globules of the oxide, melted by the heat. This oxide is a black ; 

 brittle substance, identical with hammer-scale, and different from 

 rust (ferric oxide). It contains, in fact, a smaller proportion of 

 oxygen than does the latter, and is called magnetic oxide of iron. 



Iron (167.52) + Oxygen (64) -> Magnetic oxide of iron (231.52). 



All the familiar metals, excepting gold, silver, and platinum, 

 when heated, combine with oxygen, some more vigorously, others 

 less vigorously than does iron. Oxides of the three metals just 

 named can also be made, but only by varieties of chemical change 

 other than direct combination. 



Compound substances, if they are composed largely or entirely 

 of elements which combine with oxygen, are able themselves to 

 interact with oxygen. Usually, they produce a mixture of the 

 same oxides which each element, separately, would give. Hence, 

 wood, which is composed of carbon and hydrogen with some 

 oxygen, when burnt in oxygen, produces carbon dioxide and water 

 (oxide of hydrogen) in the form of vapor. Again, carbon disul- 

 phide burns readily, giving carbon dioxide and sulphur dioxide, 

 just as do carbon and sulphur, separately. Ferrous sulphide 

 gives, similarly, sulphur dioxide and magnetic oxide of iron. 



Tests. A Test for Oxygen. A test is a property which, 



because it is easily recognized (a strong color, for example), or 

 because it is especially distinctive, is commonly employed in 

 recognizing a substance. 



Oxygen, when pure, is recognized by the fact that a splinter 

 of wood, glowing at one end, bursts into flame when introduced 

 into the gas. Only one other gas (see nitrous oxide) behaves 

 similarly. 



The Measurement of Combining Proportions. In a 



number of condensed statements we have given the proportions 

 by weight of the materials combining. It is now desirable that 



