PHYSICAL FORCES 133 



as estimated by weight according to what is known as 

 the " atomic theory" A whole series of symbols has been 

 devised to conveniently denote such conditions ; thus water 

 is imagined to be made up of molecules, each of which is 

 composed of two atoms of hydrogen and one atom of oxy- 

 gen. Water therefore is represented by the symbol H 2 O, 

 and H 2 SO 4 stands for sulphuric acid S signifying sulphur. 

 Similarly, a multitude of analogous symbols serve to 

 denote a multitude of different substances. This system 

 is of enormous practical utility, and demands the most 

 careful study, though the absolute constitution of matter 

 remains unknown to us. For farther information about 

 the laws concerning the proportions in which different 

 elements and chemical substances combine, and the sym- 

 bols made use of to denote such proportions, the reader 

 must have recourse to treatises on chemistry. 



Chemical energy is very closely related to the other 

 physical energies. It not only occasions warmth but 

 may be greatly facilitated thereby, while the heat and 

 light it may occasion can be made manifest by acting 

 as just described, with potassium. It is, however, also 

 exemplified by every fire and every candle which we see 

 burning, since every such phenomenon is both a result 

 and a sign of the exertion of chemical energy. Chemical 

 energy is most intimately related to electricity. Indeed 

 the resolution, by chemical energy, of different sub- 

 stances is always accompanied by some electrical change. 



An electric current, on the other hand, can produce 

 chemical changes in substances through which it passes, 

 and so bring about changes not as yet otherwise pro- 

 ducible, as we have just seen with regard to a spark 

 traversing a mixture of oxygen and hydrogen and so 

 evolving water. There are certain chemical substances 

 termed acids (e.g., sulphuric and nitric acid), and others 



