1872.] 



of Mixtures of Methylic Alcohol and Water. 



341 



Relation existing between the specific heat of mixtures and the heat 

 evolved during their formation : — 



1. The difference between the number of heat-units evolved during the 

 mixing of given weights of two substances, at the temperatures t and t' 

 respectively, is equal to the difference between the number of heat-units 

 required to raise the mixture, and that required to raise the two constituents 

 taken separate^, from the lower to the higher temperature. Or let U and 

 U' be the units of heat evolved by mixing x and y at the temperatures t 

 and t' respectively, S, S', and S" the specific heat of the mixture z and its 

 constituents x and y respectively, then 



U~U' = z . S (t-t')~{x . S' (t-f) +y S" (t-t')\. 



2. If more units of heat are evolved at the higher than at the lower tem- 

 perature, the specific heat of the resulting mixture will be below the 

 calculated mean ; on the other hand, the specific heat of the mixture will 

 be above the calculated mean, if the greater number of heat-units be 

 evolved at the lower temperature*. 



3. The absorption of a lesser number of heat-units will be of course 

 equivalent to the evolution of a greater number, while the absorption of 

 a greater number will be equivalent to the evolution of a smaller number of 

 heat-units. 



A series of Tables are next given showing that the following mixtures, 

 namely, ethylic alcohol and water, methylic alcohol and water, prussic acid 

 and water, ethylic alcohol and bisulphide of carbon, aqueous solutions of 

 potassium chloride, sodium chloride, potassium nitrate, and potassium 

 hydrate, conform to proposition 2, and all, except the mixture of ethylic 

 alcohol and bisulphide of carbon, also conform as closely as can be ex- 

 pected with proposition 1. Perfect agreement is only possible if the 

 specific heat of each constituent, as w 7 ell as that of the mixture, has been 

 accurately determined between the two temperatures for w r hich the heat 

 evolved on mixing has been estimated ; and this has not, in the majority of 

 cases, been done as yet. 



The author next points out that, at first sight, the above observations may 

 be explained by assuming that a difference in the chemical constitution of 

 the mixture is produced by a change of temperaturef . In some cases a 

 rise in temperature may produce a certain amount of decomposition or 

 dissociation, while a fall would be accompanied by recombination ; the 

 apparent specific heat of the mixture would thereby become raised. In 

 some other cases the reverse may happen, and the specific heat of the 

 mixture would fall below the mean. This supposition seems, however, 

 incompatible with the observed rate of expansion of mixtures, both of 

 ethylic and methylic alcohol and water. In these the rate of expansion 

 is in some of the mixtures below the mean, indicating apparently that a 

 more intimate union takes place on heating, in others the rate of expansion 



* This relation had already been suggested by Berthelot. 



f This has already been pointed out by Pfaundler, as well as by Marignac. 



