STUDY OF THE ACTIVE PART OF THE AIR 



131 



varies. The oxides of magnesium and sodium turn reddened litmus 

 blue, while those of carbon, phosphorus and sulphur turn it red. Iron 

 oxide has no action, but it does not dissolve in water. Notice that 

 the first two substances named are metals. Solutions of the oxides of 

 carbon, phosphorus and sulphur have a sour taste, and it is to this fact 

 that oxygen owes its name, which means acid-producer. It has been 

 found that the following statements are true of other oxides besides 

 those examined. 



Metallic oxides when soluble in water turn reddened litmus blue. 



Non-metallic oxides when soluble in water turn litmus red. 



OXIDES. 



Combustion. At all times the phenomenon of burning has attracted 

 attention and excited speculation as to its nature. For long the 

 idea was held that something of a material nature some sort cf fiery/ 

 matter was the cause ; but no one was successful in obtaining a> 

 specimen of such a body. 



Early in the seventeenth century Jean Key showed that the increase 

 in weight of metals when they were calcined was due to air. A little 

 later Robert Hooke proved that while charcoal might be heated very 

 strongly without burning if air was excluded, it takes fire if brought 

 into the presence of air at the same temperature. 



John Mayow (1674) burnt a candle in a closed vessel over water 

 and noticed the diminution of the volume of the air, and the difference 

 of the character of the remaining air after the candle was extinguished. 

 He concluded that air contains two kinds of particles, one of which is 

 withdrawn when a candle is burnt. 



In spite of these discoveries, which were leading up to the truth, 

 another and wrong theory was accepted as true for nearly 100 years. 

 It is difficult to understand why this happened, but it must be remem- 

 bered that at that time the balance had not come into general use in 

 chemical studies. 



