ANIMAL HEAT. 33 



bustion, can produce more heat than if directly 

 burned in atmospheric air or in oxygen gas. 



When we kindle a fire under a steam-engine, and 

 employ the power obtained to produce heat by 

 friction, it is impossible that the heat thus obtained 

 can ever be greater than that which was required 

 to heat the boiler ; and if we use the galvanic cur- 

 rent to produce heat, the amount of heat obtained 

 is never, in any circumstances, greater than we 

 might have by the combustion of the zinc which 

 has been dissolved in the acid. 



The contraction of muscles produces heat ; but 

 the force necessary for the contraction has mani- 

 fested itself through the organs of motion, in which 

 it has been excited by chemical changes. The ul- 

 timate cause of the heat produced is therefore to 

 be found in these chemical changes. 



By dissolving a metal in an acid, we produce an 

 electrical current ; this current, if passed through a 

 wire, converts the wire into a magnet, by means of 

 which many different effects may be produced. The 

 cause of these phenomena is magnetism ; the cause 

 of the magnetic phenomena is to be found in the 

 electrical current; and the ultimate cause of the 

 electrical current is found to be a chemical change, 

 a chemical action. 



There* are various causes by which force or motion 



may be produced. A bent spring, a current of 



air, the fall of water, fire applied to a boiler, the 



solution of a metal in an acid, all these different 



D 



