132 JUNIOR GRADE SCIENCE 



The phlogistic theory. This theory was first stated by Becher 

 about the middle of the seventeenth century, but not much notice 

 was taken of it till it was restated by Stahl some years later. According 

 to it, all combustible substances are composed of two parts ; one of 

 these being the same in all of them, and the other different for each. 

 The name phlogiston was given to the common substance, and burning 

 was supposed to consist in or be caused by the escape of this substance 

 from the body. Substances which after burning left a bulky ash were 

 supposed to contain very little of the phlogiston, and those which 

 burnt away leaving little or no ash were supposed to be rich in phlogiston. 

 There were many points which were difficult to explain in accordance 

 with this theory. For example ; a lighted candle placed under a bell jar 

 is extinguished long before the material of the candle is used up. The 

 explanation offered was that a given volume of air can only take up a 

 limited amount of phlogiston, when this limit is reached the air is said to 

 be phlogisticated, and no further burning can take place in it, hence the 

 candle goes out. Again, certain substances were known to increase in 

 weight when calcined. To this Stahl replied that phlogiston is "a 

 principle of levity " the presence of which in a substance causes it to 

 weigh less than when without it. 



The true theory of combustion. The discovery of oxygen led 

 Lavoisier to put forward the true theory, viz. that burning consists in 

 the combination of substances with oxygen. This theory meets all 

 difficulties, but for a time there were many l>elievers in the old theory, 

 and Priestley maintained its truth till his death, although his discovery 

 of oxygen was the cause of its overthrow. 



60. THE INACTIVE PART OF AIR. 



i. The inactive part of air. Repeat the experiment 56 i. but as soon as 

 the phosphorus becomes ignited place the jar mouth downwards over water 

 in a pneumatic trough, and satisfy yourself that the gas left behind (a) ex- 

 tinguishes a flame ; (b) has no action on a litmus paper ; (c) does not turn 

 lime-water milky. 



ii. Air can be obtained again by mixing oxygen with the inactive part 

 of air. Again burn phosphorus in an enclosed amount of air. Place the 

 delivery tube from an oxygen apparatus, similar to that used in the experi- 

 ment 58 iii., and by heating the hard glass tube bubble oxygen into the 

 jar until it is again full 'of gas. Cover the mouth of the jar with a 

 ground glass plate, lift out the jar, and test the gas it contains with a 

 burning taper : it behaves just like air. 



Nitrogen. The gas which is left in a jar of air after a metal has 

 been calcined in it, or phosphorus has burnt in it, or in which any one 

 of the instances of burning which have now been studied has occurred, 



