Dr. AVoods on tJie Heat of Chemical Combination. 269 



-8'>To me this is a new idea, and one with which I have not met 

 in any work on chemical research. I believe it will be found 

 important, and in the present paper I will confine myself to a 

 proof of its tinith. 



(2.) That decomposition generally requires a certain amount 

 of heat is admitted, for as a general rule heat causes decompo- 

 sition ; that is, to decompose a substance a source for the supply 

 of heat is necessary, or some body capable of giving up heat to 

 the compound to be decomposed must be brought into its vicinity 

 before decomposition takes place. 



(3.) That decomposition absorbs as much heat as combination 

 produces, might be proved by the fact, that in double decompo- 

 sitions no heat is given off. We know, for instance, that car- 

 bonic acid and magnesia in combining produce a certain amount 

 of heat, and that sulphuric acid and potash likewise give rise 

 to a definite quantity ; and yet when sulphate of magnesia and 

 carbonate of potash are mixed in solution, although such com- 

 binations take place, no rise of temperature (except that of soli- 

 dification) occurs. Does not the decomposition neutralize the 

 effect of the combination ? 



(4.) In cases of simple decompositions the same result does 

 not obtain. We know from the researches of Andrews and others, 

 that certain bases produce on combination with acids certain 

 amounts of heat, and that each base gives rise to a different quan- 

 tity. Andrews has also proved, that if one base displace another 

 a definite rise of temperature is the consequence. Now if the 

 rise of temperature be examined, it will be found that it is the 

 difference of the amount produced by the combining and separating 

 body. For instance, an equivalent of potash by combining with 

 an acid produces 6| units of heat; ammonia, 5^ units. If the 

 potash displace the ammonia from any of its combinations, one 

 unit of heat is the result ; the difference between 6^ units of 

 heat produced by the formation of one salt and 5| units of oold 

 by the decomposition of the other. ,/ ' r ' ,,''^. 



(5.) It occurred to me, however, that the fact might be proved 

 more satisfactorily by finding what amount of heat is given off 

 by the combustion of zinc, then ascertaining what quantity ig 

 evolved by its oxidizement in water ; for, as the water must be 

 decomposed, we should have the amount of heat in the second 

 instance less than that in the first, and less by the quantity of 

 heat produced when hydrogen is burnt. And such I find to 

 be precisely the case. 



Grassi shows that 1 lb. of oxygen uniting with hydrogen raises 

 the temperature of 43'4 lbs. of water 180°. 



The amount of heat produced, according to Despretz^ when 

 zinc is burnt, is sufficient to raise the temperature of 53 lbs. of 

 water 180°. 



Phil Mag, S. 4 Vol. 2. No. 11. Oct, 1851. U 



