CHEMICAL ACTIONS. 161 



In many instances these actions take place when some or other 

 of the substances involved are employed in the form of solutions, 

 the solvent taking no part in the chemical change, but simply 

 facilitating it by enabling the particles of reacting matter to come 

 together more freely, in virtue of the mobility peculiar to the liquid 

 state ; but the use of a solvent is by no means absolutely necessary 

 for the occurrence of chemical changes. 



In some instances the application of heat to a solid causes it to 

 melt, and the further application of heat causes the melted sub- 

 stance to decompose, the breaking up being facilitated by the 

 fluid condition assumed on melting; but this state of fluidity 

 through heat is not essential, as solid bodies may often be decom- 

 posed without any previous melting. Thus the operation of 

 burning limestone to quicklime is a case in point. When lumps 

 of limestone are strongly heated by fire (practically effected by 

 mixing the limestone and the fuel together and burning the 

 mixture in a large grate or "kiln"), they become decomposed and 

 lose largely in weight ; about -f of the material is converted into 

 gas (carbon dioxide, identical with that produced by acting on 

 chalk with hydrochloric acid, Expt. 99), which escapes, whilst the 

 remaining quicklime differs in very many respects from the original 

 limestone ; more especially in possessing the property of " slaking " 

 with water, i.e., evolving heat when wetted with water, owing to 

 combination taking place between the quicklime and the water, 

 producing " slaked lime ; " and in being somewhat soluble in water, 

 yielding an alkaline solution, or "lime water" (Expts. 152 and 

 240). 



On the other hand, when metals are heated in contact with air, 

 and more especially when heated in an atmosphere of pure oxygen, 

 they usually (though not invariably) combine with the oxygen 

 present, producing "oxides" (Expts. 15, 18). Sometimes the 

 combination is attended with the development of great heat and 

 vivid light, i.e., vigorous "combustion" is brought about, just as 

 when coke or coal burns in the air, owing to the combination of 

 the oxygen with the oxidisable or combustible matter present. 

 Here, obviously, no solvents are employed. 



Yet again, if certain oxides, e.g., the oxide of lead thus produced, 

 be mixed with powdered charcoal, and strongly heated in a crucible 

 so that the external air does not get access to the mass, the char- 

 coal will act upon the oxide of lead in exactly the same kind of 

 way that zinc acts upon hydrochloric acid (Expt. 10) ; the char- 

 coal will take away the oxygen from the oxide of lead, forming 

 metallic lead, and producing oxide of carbon, just as the zinc takes 

 the chlorine away from the hydrogen, producing free hydrogen and 



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