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There are two aspects of a chemical reaction such as the 

 combination of carbon and oxygen now under consideration. One 

 is the material aspect, the other the energetic aspect. From the 

 point of view of human convenience sometimes the one, some- 

 times the other, is the more important. Consider these two 

 equations : — 



CaO + H 2 0 = Ca(OH) 2 + Qi (units of energy). 



C + 0 2 = CG 2 + Q 2 (units of energy). 



The first refers to the action of water on quick-lime. Slaked 

 lime is the material product, heat is evolved. When the builder 

 slakes lime he does so because he wants the material product, the 

 slaked lime, to use in making mortar. The evolution of heat is 

 to him of quite secondary importance. It may assure him that 

 his lime is of good quality, but otherwise is of little interest to 

 him. 



The second equation refers to the burning of carbon. Carbon 

 dioxide is formed ; this is the material product, but energy, chiefly 

 in the form of heat, is simultaneously set free. When the French- 

 man burns charcoal in a brazier or we burn coal or coke, it is done, 

 not usually for the sake of the material product, but for the 

 immaterial one, the heat that is evolved. In burning candles, our 

 object is not to produce carbon dioxide and water, but to get the 

 benefit of another form of energy, namely, light. The combustion 

 of carbon, or of carbon-containing substances, is our principal 

 means of obtaining not only heat, but useful electrical and 

 mechanical energy. Thus it is the energetic rather than the 

 material aspect of our second equation which is of the greater 

 practical importance to us. 



There are many other ways in which carbon dioxide is pro- 

 duced, among these may be mentioned — 



1. It is formed in the respiration of animals. 



2. Processes analogous to animal respiration occur in the 

 case of plants. During the germination of seeds, for 

 instance, there is an elevation of temperature and a form- 

 ation of carbon dioxide. 



3. During fermentation large amounts are set free. 



4. The action of heat on carbonate of lime and most other 

 carbonates, that of sodium being one of the exceptions, 

 liberates the gas. 



5. By acting on carbonates with acids carbon dioxide is 

 readily and conveniently produced. 



Kipp's apparatus, when charged with marble and diluted 

 hydrochloric acid, is very suitable for preparing the gas for ex- 

 perimental purposes. With such an apparatus we can easily show 

 some of the chief properties of the compound such as its density, 

 about 1^ times that of air; its action in extingu'shing flame, pre- 

 cipitating lime-water and reddening blue litmus. 



Carbon dioxide exists in the air to the extent of 3.5 or 40 

 volumes in 100,000. In the narrow streets of large towns and 



