CARBONIC ACID. 277 



Q. Whence does the HEAT of FIRE arise ? 



A. The carbon of fuel (when heated) 

 combines with the oxygen of the air, and 

 produces carbonic acid gas: again, the 

 hydrogen of the fuel combining with other 

 portions of oxygen, condenses into water ; 

 by which chemical actions heat is evolved. 



Q. Whence does the HEAT of our own BODY 

 arise ? 



A. The carbon of the blood combines 

 with the oxygen of the air inhaled, and 

 produces carbonic acid gas ; which pro- 

 duces heat in a way similar to burning 

 fuel. 



Q. Whence does the HEAT of a DUNGHILL 

 arise? 



A. The straw, fyc. of the dunghill 

 undergoes fermentation as it decays : the 

 fermentation produces carbonic acid gas, 

 and heat is evolved by a species of com- 

 bustion (as in the two former cases). 



Q. What changes do VEGETABLES undergo 



from PUTREFACTION ? 



A. The hydrogen of the vegetables 

 combines with the oxygen of the air, and 

 forms water: again, the carbon of the 



Ba 



