450 



REPORT — 1886. 



given contraction.^ The contraction whicli ensues when H2SO4 is 

 diluted with H2O amounts to 5-814< unit volumes upon 53-203 + 18, or 

 71-203, which is the sum of the molecular volumes of these liquids. And 



Fig. 1. 



'-ETCrO/eOO 

 I MOLS 



Vt^ 



MOLECULES or w/irrR 



taking the coefficient of expansion of H2SO4H2O to be '00056 for 1° C, 

 and its specific heat -442, we find that this would correspond to an evolution 

 of heat amounting to 4588-9 calories. The evolution of heat actually- 

 observed being 6379 according to Thomsen, there is a difierence of 1790 

 heat units, which is unaccounted for on the hypothesis that the change of 

 volnme is due to just such a change in the mean distances and motions 

 of the molecules of the liquid as would be brought about by a change of 

 temperature, and this may really indicate some kind or some amount of 

 chemical combination. 



The majority of solids, and nearly all salts, expand on being melted. 

 And as, whatever theory we adopt as to the constitution of solutions, we 

 must admit that the dissolved substance is in the liquid state, it is obvious 

 that in all cases where a solid is dissolved by a liquid the change of 

 volume which is observed must be the resultant of the two distinct 

 changes of volume consequent upon the (1) liquefaction of the solid, and 

 (2) the intermixture of the resulting liquid with the solvent. 



The diagram (fig. 1) already given exhibits the amount of contraction 

 observed in the case of sulphuric acid and water, and the following table 

 shows the observed and calculated molecular volumes by which the amount 

 of contraction is indicated. 



' Favre & Valson, Compt. Bend. Ixxvii. ; Jaliresh. 1872. 



