20 COMBUSTION. 



densed by mixture heat is evolved ; and when they be- 

 come more expanded cold is produced. If one ounce of 

 sulphuric acid be mixed with four ounces of cold water, 

 a degree of heat greater than that of boiling water will 

 be produced ; but if the mixture be measured, it will be 

 found, for the above reason, not to amount to so much 

 as five ounces. Bodies, in changing from a liquid to a 

 solid form, always evolve heat. In slaking lime, or in 

 mixing plaster of Paris, a considerable degree of latent 

 caloric is evolved, from the water changing its fluid for 

 a solid state. Water, in becoming ice, gives out its la- 

 tent caloric, and that so freely as to render the progress 

 of freezing comparatively slow. 



Electricity will also produce caloric ; but the most in- 

 tense heat known has been obtained from galvanism. 

 By means of the galvanic battery of the Royal Institu- 

 tion the most powerful and surprising effects have been 

 produced. To say nothing of the metals, quartz, the 

 sapphire, magnesia, fragments of the diamond, charcoal, 

 plumbago, &c., seem to evaporate and disappear rapidly 

 under its influence. The most insoluble compounds, 

 the most compact substances with which we are ac- 

 quainted, are by its means decomposed and resolved 

 into their simplest forms. 



COMBUSTION. 



Simple and Compouud Combustibles Supporters of Combustion Theory of 

 Combustion Result of Combustion Combustion simply Decomposition 

 Sir Richard Phillips's Theory "of Combustion Different states of Combus- 

 tion, &c. 



COMBUSTION is that evolution of heat and light which 

 accompanies certain chemical combinations and decom- 

 positions. Until late years the nature of combustion 

 was not at all understood ; and even at present it can 

 not be said to be perfectly known. 



All bodies may be divided into two kinds, combustibles 

 and incombustiblcs. Combustible bodies are again di- 

 vided into simple and compound. Simple combustibles are 

 those which cannot be decomposed ; they are hydrogen, 

 (the base of water), carbon (the base of charcoal), sul- 



