126 JUNIOR GRADE SCIENCE 



air, and unite with it to form fresh substances, it should not be difficult 

 to make these, or similar substances, give up the part of the air which 

 they take up, and so procure the active constituent of air by itself in 

 a pure form. But a little more thought suggests that probably some 

 of these substances would do better than others. It is quite certain 

 that some are formed more easily than others. Will those which are 

 most easily formed be the best from which to get the active part ? 

 No. The reason is this. When a chemical change takes place easily 

 it generally means that the substances taking part in the change have, 

 as i f . were, a great liking for one another, and when they combine 

 together they form a compound which it is difficult to separate into 

 its parts again. The easiest way to set to work is, therefore, first to 

 find some substance which only combines with the active part slowly 

 and with difficulty, for the compound such a substance forms with the 

 active part will most likely be a weak one, and easily broken up again. 



The compound which lead forms with the active part of the 

 air. When lead is heated in contact with the air, a yellow powder 

 which is much darker in colour when hot is formed. If the heating 

 is continued long enough, all the metal is changed into powder. The 

 change takes place fairly easily, so that from previous reasoning it 

 may be concluded that it is probably difficult to get the active part 

 of the air again from this powder. This is so. But it is found that 

 when some of the yellow powder is heated for a long time at the tem- 

 perature at which lead melts, it slowly takes up still more of the active 

 par*, of the air, and changes in colour, becoming red. The first powder 

 obtained, which is yellow, is in some states called litharge ; the second 

 red powder is known as red lead. It is easy to get the second lot of 

 the active part of the air again from the red lead. A third powder 

 of a black colour, and having less of the active part of air than either 

 litharge or red lead, is also formed when lead is heated in air. 



How the active part of air is obtained from red lead. When 

 red lead is heated it changes in colour, and if the heat has not been 

 great it regains its original red colour when allowed to cool. But if 

 strongly heated the red lead gives up some of the active part of the 

 air which it contains, and is reconverted into litharge. The amount 

 of the active part of the air which it thus gives up on being heated is 

 the second quantity referred to in the last paragraph, which is taken 

 up slowly when the heating of lead is continued for a long time. If 

 red lead is strongly heated in a tut>e, as in Fig. 94, and a glowing 

 splinter of wood is pushed down the tube, the splinter bursts into flame 

 and continues to burn brightly. The active part of air has been 

 obtained alone, and supports burning strongly. 



