52 PRINCIPLES OF CHEMISTRY. 



For instance: A piece of marble, which is carbonate of cal- 

 cium, or CaC0 3 , is decomposed by heat into oxide of calcium, 

 CaO, and carbon dioxide, C0 2 . 



The amount of heat required for decomposition differs widely 

 according to the nature of the substance. Some substances can 

 only be produced at a temperature below the freezing-point of 

 water, a higher temperature causing their decomposition ; other 

 substances will be decomposed at temperatures between the 

 freezing- and boiling-points ; others again, and to these belong 

 the majority of compounds, may be raised to red or white heat 

 before decomposition sets in ; and still another number of com- 

 pounds have never yet been decomposed by heat. Theoreti- 

 cally, however, we assume that all compounds may be decom- 

 posed by heat, should it be possible to raise it to a sufficiently 

 high degree. 



Decomposition by electricity. Similarly to heat, electricity also 

 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. 

 There is a certain relation between electrical and chemical 

 action, as the amount of electricity which for instance sets free 

 35.5 grams of chlorine, will also set free 80 grams of bromine 

 or 127 grams of iodine. The figures 35.5, 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 hypo- 

 thetical medium sether ; electricity is the consequence of the 

 motion of an assumed electric fluid. 



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



