54 PRINCIPLES OF CHEMISTRY. 



points ; others again, and to these belong the majority of inorganic 

 compounds, may be raised to red or white heat before decomposition 

 sets in ; and still another number of compounds have never yet been 

 decomposed by heat. / Theoretically , however, we assume that all 

 compounds may be decomposed, by heat, should it be possible to raise 

 it to a sufficiently high degree. ] 



Decomposition by electricity. Similarly to heat, also electricity 

 decomposes many substances, provided they are in a liquid or gaseous 

 state. These decompositions are usually accomplished by allowing an 

 electric current to pass through the liquid, or electric sparks to pass 

 through the gas. Thus hydrochloric acid, HC1, may be decomposed 

 into hydrogen and chlorine; water, H^O, into hydrogen and oxygen. 



The act of decomposing a compound by electricity is known as electrolysis, 

 and the substance thus decomposed is termed electrolyte. During the decom- 

 position of substances by electrolysis one of the products of decomposition 

 appears at the negative, the other at the positive pole of the battery. Trnn, 

 when water is decomposed, the hydrogen is evolved from the negative, the 

 oxygen from the positive pole. Or, when salts are decomposed, the metal is 

 deposited at the negative pole, and the acid, or its decomposition products at 

 the positive pole. 



It was formerly believed that those elements which in electrolysis appear at 

 the negative pole were charged with positive electricity, and were called electro- 

 positive elements, while those appearing at the positive pole were charged with 

 negative electricity and called electro-negative elements. According to this view 

 the non-metals are electro-negative, while the metals are electro-positive. 



There is a certain relation between electrical and chemical action, as 

 the quantity of electricity which, for instance, sets free 35.4 grammes 

 of chlorine, will also set free 80 grammes of bromine or 127 grammes 

 of iodine. The figures 35.4, 80, and 127 represent the atomic weights 

 of these elements. 



Decomposition by light. Another cause of decomposition is, in 

 many cases, the action of light. The art of photography is based 

 upon this kind of decomposition. Many substances, easily affected 

 by light, have to be kept in the dark to prevent them from being 

 decomposed. 



The phenomena of heat, light, and electricity resemble each other in so far 

 as they are phenomena of motion. Heat is the consequence of the motion of 

 material particles (molecules); light is the consequence of the vibratory motion 

 of the hypothetical medium ether ; probably the same is true of electricity. 



These motions, in being transferred to atoms, have, as shown above, frequently 

 the tendency of splitting up the molecules of compound substances. 



