270 Dr. Woods on the Heat of Chemical Combinations 



I find that 1 gi*. of zinc, by dissolving in dilute sulphuric acid, 

 raises the temperature of GO grs. of water 18*^ F., making the 

 nccessaiy allowance for specific heats, containing vessel, &c. ; but 

 1 gr. of zinc is equivalent to 1 j gr. of oxide of zinc, and this 

 would, by its combination with the acid, raise the temperature 

 of CO grs. of water 10^° (Graham, Andrews). This amount 

 must therefore be taken from the 18" in order to find what is 

 due to the oxidizemcnt of the zinc alone. One grain of zinc, there- 

 fore, by being oxidized, or \ gr. of oxygen uniting with zinc with 

 decomposition of water, raises the temperature of 60 grs. of water 

 7^° F. This is the same as 1 lb. of oxygen uniting with zinc, 

 with decomposition of water raising the temperature of 10 lbs. of 

 water 180^, whereas without the decomposition it raises the tem- 

 perature of 53 lbs. of water 180°; therefore the decomposition 

 absorbs as much heat as would raise 43 lbs. of water 180° — the 

 same amount which we saw the combination of oxygen and hy- 

 drogen was capable of producing. 



(6.) If zinc be dissolved in muriatic acidy I find that 1 gr. 

 causes a rise of temperature equal to 21° F. in 60 grs, of water ; 

 or 1 lb. of chlorine uniting with zinc, with decomposition of 

 chloride of hydrogen, raises the temperature of 6 lbs. of water 

 180°. Abria {Ulnstiiut, vol. xiv. p. 635) found that 1 lb. of 

 chlorine uniting directly with zinc would raise the temperature 

 of 36 lbs. of water 180 ; that is, y%ths of what oxygen produces. 

 If chlorine uniting with zinc produces likewise y^ths of the heat 

 oxygen does under similar circumstances, 4.2 lbs. of water would 

 be raised 180° by 1 lb. uniting with the metal. In other words, 

 zinc uniting with chlorine tvith decomposition of chloride of hy- 

 drogen produces 6 units of heat, without decomposition 42 units. 

 The difference, or 3Q units, is what the combination of chlorine 

 and hydrogen produce, and consequently what is lost by the de- 

 composition. 



(7.) Wlien 1 lb. of zinc is dissolved in nitric acid, it raises 

 the temperature of 43 lbs. of water 180° F., more than twice as 

 much heat being produced than when it is dissolved in sulphuric 

 acid. This combination is accompanied by decomposition of 

 nitric acid. 



The last two instances are brought forward for the present 

 only to prove, that it is not because zinc is oxidized under dif- 

 ferent circumstances, as in air and water, that different amounts 

 of heat are produced, but that an absorption of heat always 

 accompanies decomposition, and varies with the substances de- 

 composed. The first instance, the solution of zinc in sulphuric 

 acid, proves that this absorption is equal to the quantity liberated 

 when the same elements combine. 



(8.) When I had satisfied myself so far, I thought that if a 

 compound body could be decomposed directly, or without com- 



