291 



with its liquid, was 6rst investigated by the author in the third sec- 

 tion of the paper. 



In article 70 there is deduced from the new form of the thermody- 

 namic function, a law called that of the " Total Heat of Gazefica- 

 tion,'^ which includes, as a consequence, the law of the total heat of 

 evaporation. The total heat of gazeficatxon of a given substance, 

 under constant pressure, between two given temperatures, is the 

 heat which must be communicated to the substance in order to con- 

 vert it from the liquid or solid state at the lower temperature, to 

 the state of perfect gas at the higher temperature, — evaporation 

 taking place at the boiling point corresponding to the constant 

 pressure under which the whole operation is performed. When the 

 bulk of the substance (as is the case for all known substances) is very 

 small in the liquid or solid state, as compared with its bulk in the 

 state of perfect gas, the total heat of gazefication, under constant 

 pressure, between two given temperatures, does not sensibly vary 

 with the pressure. 



This law is of great importance in connection with the employ- 

 ment of super-heated vapours to drive machinery. 



In article 71 are given formulae, founded on the experiments of 

 M. Regnault, for computing the pressures of the vapours of aether, 

 bi-sulphuret of carbon, alcohol above 0° c, water, essence of turpen- 

 tine above 40° c, chloroform above 70° c, and mercury up to 358° c. 

 The table of constants for these fluids is extracted from a paper read 

 before the British Association in September 1854, and published 

 in the Philosophical Magazine for December 1854. 



In article 72, it is shown how these formulae are applied to 

 calculate the latent heat of evaporation for unity of space. 



In article 73, it is stated, that if the latent heat of evaporation of 

 unity of weight of a fluid be known by experiment for a given tem- 

 perature of ebullition, and the latent heat of evaporation for unity 

 of space be computed theoretically, the volume of unity of weight of 

 the vapour at the given temperature of ebullition may be calculated 

 from these data. This principle is applied to the latent heats of 

 evaporation, under atmospheric pressure, of aether, sulphuret of car- 

 bon, and alcohol, as determined experimentally by Dr Andrews, and 

 of water, as determined by M. Regnault. The results of these 

 calculations are compared with those of computations founded on the 

 chemical composition of the fluids, and the supposition that their 



