122 PRACTICAL LESSONS IN SCIENCE. 



lence under the influence of direct sunlight, taking place slowly 

 and quietly in moderately strong, diffused light. The action of 

 sunlight in this and other cases of chemical action, as in photog- 

 raphy, is not well understood. It does not seem to be the heat, 

 as even a higher degree of heat without light has no such effect. 

 Compounds of oxygen with elements are called oxides, the com- 

 pounds of chlorine with elements are called chlorides. 



Chlorine combines energetically with bromine, iodine, sulphur, 

 phosphorus and arsenic, and with most of the metals also, mak- 

 ing it the rival of oxygen in the extent of its combinations. Chlo- 

 rine forms several interesting compounds with hydrogen and 

 oxygen. They are, hypochlorous acid, HC10; chlorous acid, 

 HC10 2 ; chloric acid, HC10 3 ; perchloric acid, HC10 4 . 



The chloric and hypochlorous acid are the most important, 

 but the whole series is interesting as illustrating the law of mul- 

 tiple proportions. Chloric acid may be prepared by treating po- 

 tassium chlorate with sulphuric acid. The reactions are as 

 follows : 2KC10 3 +H 2 S0 4 =K 2 S0 4 +2HC10 3 . The potassium tak- 

 ing the place of the hydrogen in the sulphuric acid forms po- 

 tassium sulphate, and the hydrogen with the chlorine and 

 oxygen forms the chloric acid. The chloric and hypochloroun 

 acids form salts as the calcium hypochlorite, usually called 

 chloride of lime, or bleaching powder, and the potassium chlorate. 

 These compounds of hydrogen, chlorine and oxygen are not very 

 stable. The affinity with oxygen is not strong. 



In considering the composition of hydrochloric acid it is found 

 that one volume of hydrogen unites with one volume of chlorine 

 to form two volumes of hydrochloric-acid gas. In the compo- 

 sition of water, two volumes of hydrogen combine with one 

 volume of oxygen forming two volumes of water-vapor. In the 

 case of ammonia, three volumes of hydrogen combined with one 

 of nitrogen to form two volumes of ammonia, and in marsh gas 

 we have four volumes of hydrogen combined with one of carbon 

 into two volumes of marsh gas. 



These facts were discovered and have been abundantly verified 

 by experiment ; but how are they explained ? Dr. Dalton formu- 



