149 



Gases. 



Oxygen. 



The air contains constantly oxygen, nitrogen, argon, carbonic 

 acid, and vapor of water. The first two make up the main bulk 

 of the air, the last three are very small in amount. We wish to 

 study each of these gases, except argon, a recently discovered 

 one, which in this work we cannot make or isolate. Oxygen can 

 be made and studied with very simple apparatus. 



The materials used are potassium chlorate and black oxide of 

 manganese. The potassium chlorate gives up the oxygen it con- 

 tains very readily on heating. In fact it is liable to give off such 

 a large quantity of gas at once, as to produce an explosion. Con- 

 sequently we mix with it the black oxide of manganese, which 

 seems to retard the giving off of the oxygen. 



Mix well equal amounts of the two substances (fig. 81.) A 

 test tube one-third full will make sufficient gas for the work. Fit 

 the test-tube with an air tight cork and a glass tube to carry off 

 the gas. To catch the gas, have ready at least five wide-mouthed 

 bottles (8 to 15 oz. in size). These are filled with water and 

 inverted in a pan of water. The delivery-tube carrying the gas 

 from the test-tube is bent so that it can be made to conveniently 

 reach under the mouth of an inverted bottle. When all is ready, 

 with the alcohol lamp heat the potassium chlorate and black 

 oxide of manganese mixture and the gas will rapidly come away 

 and bubble up into the inverted bottle, displacing the water and 

 filling the bottle. Then another bottle is brought over and so on, 

 till all are filled, or the oxygen gives out. 



In beginning to apply the heat, do so at first gently and to 

 the upper part of the test-tube. This will heat the tube and pre- 

 vent moisture forming on it later and breaking the tube. Next 

 heat the upper part of the potassium chlorate mixture, first ex- 



