SCIENCE OF COMMON THINGS. 137 



Heat of electricity. Heat of chemical action. 



to another, the substance which serves to conduct it is 

 very frequently heated ; but in what manner the heat 

 is produced, we have no positive information. 



8 7O How great a degree of heat is electricity capable of generating f 



The greatest known heat with which we are acquaint- 

 ed, is produced by the agency of the electric or gal- 

 vanic current. Ah 1 known substances can be melted or 

 volatilized by it. 



871 Has the heat generated by electricity been employed for any prac- 

 tical or economical purposes ? 



Not to any great extent ; but for philosophical experi- 

 ments and investigations it has been made quite useful. 



873 What is chemical action ? 



We apply the term chemical action to those opera- 

 tions, whatever they may be, by which the weight, form, 

 solidity, color, taste, smell, and action of substances 

 become changed ; so that new bodies with quite different 

 properties are formed from the old. 



873 How does chemical action become a source of heat ? 



Many bodies, when their original constitution is 

 altered, either by the abstraction of some of their com- 

 ponent parts, or by the addition of other substances not 

 before in combination with them, evolve heat while the 

 change is taking place. 



874 Explain by illustration what you mean. 



"Water is cold, and sulphuric acid is cold ; but if these 

 two cold liquids be mixed together, they will produce 

 intense heat. 



875 Why does cold water poured on lime make it intensely hot ? 



Because heat is evolved by the chemical action which 

 takes place when the cold water combines with the lirne. 



Heat is always evolved when a fluid is converted into a solid form. 

 Heat is always absorbed when a solid is changed into a liquid state. As 

 the water is changed from its liquid form when it is taken up by the lime, 

 therefore heat is given off. 



87Q Where does the heat come from f 



It was in the water and lime before, but was in a 

 latent state. 



