72 ANIMAL CHEMISTRY LECTURE IV. 



granulated zinc in hydrochloric acid, now taking place in this 

 gas-generating flask, is being accompanied by a considerable 

 elevation of temperature in the liquid, as well as by an evolution 

 of hydrogen, according to the equation 



Zinc Hydrochloric Zinc chloride Hydrogen 



Zn" + 2HC1 = Zn"Cl z + H a . 



(76.) Now, equivalent for equivalent, the quantity of heat 

 liberated by the combination of zinc with oxygen or chlorine is 

 much greater than that evolved by the similar combination of 

 hydrogen ; and accordingly, when we burn zinc at the expense 

 of hydrogen, as in this experiment, we obtain in the flask just so 

 much of the heat produced by the burning of the zinc as is in 

 excess of the heat absorbed in the unburning, so to speak, of the 

 hydrogen.* Hence, leaving out of consideration certain sub- 

 sidiary phenomena, the heat produced by the solution of a given 

 quantity of zinc in hydrochloric acid, and the heat producible by 

 burning the thereby liberated hydrogen in an atmosphere of 

 chlorine, added together, would exactly equal the amount of heat 

 producible by burning the same quantity of zinc directly in 

 chlorine gas, as we did a minute or two ago. Thus, by the 

 solution of zinc in hydrochloric or sulphuric acid, we have a 

 certain proportion of the combination-heat of the metal set free. 

 The intensity of this heat is not great, in, consequence of its 

 being associated with so large a mass of matter in the flask, but 



* Vide Professor Williamson's lecture On the Dynamics of the Galvanic 

 Battery.' Phil. Mag. xxvi. 452. Taking as our unit of heat the quantity 

 of heat necessary to raise the temperature of a kilogramme of water i C. 

 that is to say, from o C. to i C. it is found that 65 grammes of zinc 

 Zn", in combining with twice 35-5 grammes of chlorine Cl a , to form 136 

 grammes of chloride of zinc Zn"Cl 2 , evolve 101-31 units of heat; whereas 

 2 grammes of hydrogen H z , in combining with twice 35-5 grammes of 

 chlorine Cl ? , to form 73 grammes of hydrochloric acid 2HC1, evolve only 

 47-56 units of heat. Hence in decomposing 73 grammes of hydrochloric 

 acid by 65 grammes of zinc, according to the equation Zn" + 2HCl = Zn"Cl 2 

 + H a , we should have 101-31 47-56 = 53-75 units of heat liberated as 

 initial battery power. 



