LESSONS IN CHEMISTRY. 107 



compound was decomposed into its original elements. The red 

 oxide of mercury is represented in chemistry by the symbol 

 HgO, which signifies that it is a compound whose molecule con- 

 tains one atom of mercury and one atom of oxygen, and the 

 proportion of the components by weight is 200 parts of mercury 

 and 16 parts of oxygen. 



The chemical formula of potassium chlorate from which 

 oxygen is usually prepared for laboratory purposes is KC10 3 , 

 which indicates a compound body whose molecule is com- 

 posed of one atom of potassium, one atom of chlorine and 

 three atoms of oxygen; by weight there are 39 parts of 

 potassium, 35.5 parts of chlorine and 48 (16X3) parts of oxy- 

 gen. Potassium chlorate however made, wherever found al- 

 ways contains these substances, and in the proportions named. 

 The decomposition may be represented by an equation as 

 KC10 3 , after heating strongly equals KC1, potassium chloride, 

 +0 8 , the atoms of oxygen. The KC1 a solid, remains in the tube 

 or retort, and the oxygen is collected in jars over water. 



The heat, strong enough to separate all the oxygen from the 

 original compound, was not sufficient to separate the chlorine 

 from the potassium. This shows that the attraction of the 

 potassium for the chlorine is stronger than the attraction of the 

 two, as combined, for the oxygen, which illustrates the general 

 fact that the elements differ widely in their attractions for each 

 other, which is a prominent feature in all chemical reactions. 



In preparing oxygen in quantity a glass or metal flask of ap- 

 propriate size is used, and the gas is collected in jars over water, 

 or in regular gas receivers. The jars are immersed in water, then 

 inverted and placed on a perforated shelf so adjusted that the 

 mouth of the jar will be just below the surface of the water. The 

 delivery tube conducts the gas under the mouth of the jar, which, 

 rising through the water crowds it out, occupying its place in the 

 jar. When wanted for experiment, a piece of glass or board may 

 be slipped under the mouth of the jar, which may then be in- 

 verted. As the gas is heavier than air it can be retained in the 

 jar for some time, even though it be covered loosely. 



