xv THE ATMOSPHERE 223 



EXIT. 221. The student may verify this by trying the 

 different acids in the laboratory. 



Acids are turther characterised by a sour taste, even when in 

 dilute solution, and by other far more important chemical proper- 

 .ties. The solution of oxide of sodium is what is called an alkaline 

 solution. The student will do well to note that, as a general 

 rule, he may regard the oxides of metals as different in their 

 properties from the oxides of the non-metallic elements. 



We may now write : 



Iron and oxygen form oxide of iron. 

 Phosphorus and oxygen form phosphorus pentoxide. 

 Carbon and oxygen form carbon dioxide. 

 Sulphur and oxygen form sulphur dioxide. 

 Sodium and oxygen form sodium oxide. 



The meaning of the term di, &c. , will be understood later. 

 We must now return to the flask which contains the residue 

 of the heated mass. 



EXPT. 222. Heat well, till no more oxygen is given off. Boil 

 up the mass with water and filter it. The manganese dioxide 

 remains unchanged on the filter paper. Evaporate the solution 

 to dryness, and there remains a white solid which is not potas- 

 sium chlorate and which tastes something like salt. 



This substance is called potassium chloride, and we may 

 therefore write : Potassium Chlorate, when heated, decom- 

 poses into potassium chloride and oxygen. The manganese 

 dioxide did not change ; what, then, was its role ? This ques- 

 tion is one which was for a long time explained by calling the 

 manganese dioxide a catalytic agent, and such actions, viz. , those 

 accelerated by the presence of a substance, which itself does not 

 change, w T ere called catalytic actions. In reality, however, the 

 manganese dioxide goes through a series of changes, but the 

 final one leaves it in its original condition. 



Quantitative Character of Chemical Action. The heating 

 of potassium chlorate and manganese dioxide must also be 

 done quantitatively, as the numbers obtained will be of use to 

 us in our later work. 



EXPT. 223. Procure a test-tube and put in it a little man- 

 ganese dioxide, and place near the mouth a plug of glass 



