110 PRACTICAL LESSONS IN SCIENCE. 



of conservation of energy holds in chemical as well as in physical 

 phenomena. 



Combustible substances are endowed with potential energy; 

 they can do work. The carbon of the coal, uniting with oxygen 

 generates heat which converts the water into steam, whose ex- 

 pansive force acting through machinery does manifold kinds of 

 work. The carbon dioxide formed by the respiration of animals 

 by many kinds of slow combustions has no potential energy, 

 but plants under the influence of sunlight can separate the carbon 

 from the oxygen and the carbon stored up in wood, coal, oil, or 

 gas, has acquired potential energy, can do work. Potential en- 

 ergy, derived from the sun, stored up in coal, oil and gas, millions 

 of years ago, is now doing a great part of the work of the world. 

 This is an interesting illustration of one of the wonderful econo- 

 mies of nature. 



The compounds of oxygen with other elements are called ox- 

 ides. Thus the union of oxygen and mercury forms mercuric 

 oxide, composed of one part of mercury to one part of oxygen. 

 In some compounds, called dioxides, there are two atoms of oxy- 

 gen, as the carbon dioxide, and sometimes there are trioxides, as 

 in the case of sulphur. In the case of copper there are two oxides, 

 whose formula are Cu 2 0,the oxide of copper, or cupric oxide, 

 and CuO, suboxide of copper, or cuprous oxide. 



HYDROGEN is a colorless invisible gas having neither odor nor 

 taste, and is the lightest substance known. Its symbol is H and 

 its combining weight I. Its weight is considered as the unit of 

 the system of combining weights. It has been found free in vol- 

 canic gases, but usually exists in combinations with other sub- 

 stances, making up one-ninth of the weight of water, and forming 

 a constituent part of all vegetable and animal bodies. Hydro- 

 gen is in many respects the most interesting of all the elements. 

 It unites with oxygen to form water, and by the decomposition 

 of water we may obtain both of these gases. 



Water may be decomposed by the voltaic current, oxygen be- 

 ing given off at the positive pole and hydrogen at the negative. 



