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L. The Heat of Combination of Substances in the Liquid 

 and Solid Condition. By Spencer Umfreville Pickering 

 F.R.S* 



IF it were found that the heat of formation of a solid 

 hydrate, or analogous compound, from its solid con- 

 stituents were the same as that evolved when these constituents 

 were mixed in the liquid condition, we should have a strong 

 argument in favour of the view that the same substance was 

 formed in the two cases — that the liquid mixture, just as. 

 much as the solid hydrate, consisted of a definite compound. 

 The only instance in which sufficient data exist for the 

 calculation of the heat of formation in the two conditions 

 gives results indicating that this may be the case : taking the 

 author's values for the monohydrate of sulphuric acid, it is 

 found that solid water and solid sulphuric acid in combining 

 to form the solid monohydrate evolve 6533 cal. at 17°*9, 

 whereas the liquid constituents in combining to form the 

 liquid hydrated acid at the same temperature evolve 6667 cal., 

 practically the same amount of heat. 



The present determinations were made in order to see 

 whether a similar equality held good in other cases. The 

 results obtained, however, have been of an entirely negative 

 character, and show that the equality found in the case of 

 sulphuric acid is, probably, accidental. 



Negative results, how r ever, do not in any way prove that 

 the mixed liquids do not contain, or consist of, the compound 

 known in the solid condition, for the actions concerned are 

 complex, and the quantity which is measured as the heat of 

 combination represents, in reality, the difference between this 

 quantity and several others. 



Thus, suppose, for the sake of argument, that the same 

 amount of combination occurs when the substances are 

 brought together in either of the two conditions : and let C 

 represent the heat of combination of the molecules a and b to 

 form ab. Then we have, when the substances are all liquids, 

 the splitting up of the liquid aggregates into the molecules 

 a and b, v a and i>j, and the aggregation of the molecules 

 of the compound into the liquid condition, so that the heat 

 measured on mixing the liquids will be 



When the substances are in the solid condition, we shall have 

 similarly the heats of conversion of the solids into the mole- 

 cular condition, or, since the action may be regarded as 

 * Communicated by the Author. 



