132 ELEMENTS OF SCIENCE 



different binary compound. Thus water, as we have 

 seen, consists of two elements, oxygen and hydrogen, 

 chemically united, and they are in the proportion of two 

 volumes of hydrogen for one volume of oxygen. Now a 

 very light metal, known as potassium, and oxygen, have 

 an extreme chemical affinity for each other. This is so 

 great that if a piece of potassium be thrown into water 

 the chemical energy thus developed is of such intensity 

 that heat and light are produced in the water, by the 

 energetic junction of some of the oxygen of the water 

 with the metal, which burns (so forming potash or oxide 

 of potassium), while such of the hydrogen of the water 

 as is thus abandoned by its oxygen, being set free, 

 also burns in uniting energetically with the oxygen in 

 the air, so forming aqueous vapour. Another such 

 reciprocal interchange of elements will take place if 

 we put together the two substances respectively known 

 as " nitrate of silver " and " hydrochloric acid." Nitrate 

 of silver is resolvable into silver, nitrogen and oxygen, 

 there being, by weight, three times as much oxygen as 

 either of the others. The hydrochloric acid consists of 

 the gases chlorine and hydrogen. When these two sub- 

 stances are placed in proximity, the chlorine will leave 

 the acid to unite with the silver of the nitrate of silver, 

 and so produce what is called chloride of silver, while 

 the hydrogen of the acid unites with the nitrogen and 

 oxygen of the nitrate of silver and so forms nitric* acid. 

 It is practically most useful, in the study of chemistry, 

 to regard each substance as made up of minute particles 

 called " molecules" and these again of still more minute 

 particles termed "atoms" combined in definite proportions 



* As to "nitrates," "carbonates," "sulphates," "phosphates," 

 and for other examples of chemical change, see^os ; pp. 139-142. 



