Maximum Pressure and Latent Heat of Vapours, 533 



Essence of Turpentine has been discovered by M. Regnault to 

 undergo a molecular change by continued boiling. For this 

 fluid the agreement between the formula and the experiments is 

 satisfactory above 40° Centigrade, and up to the limit of the 

 experiments, 222°, but not below 40°. 



It is impossible to obtain Chloroform free from an admixture 

 of foreign substances. Accordingly, M. Regnault has found 

 that the two methods of determining the pressure of the vapour 

 of this fluid give widely difi*erent results, neither of which can be 

 represented accurately by the formula now proposed below the 

 temperature of 70° Cent. From this temperature, however, up 

 to 130° Cent., the limit of the experiments, the agreement is 

 close. 



10. In the cases of alcohol and turpentine, the discrepancies 

 between the formulae and the experiments at very low tempera- 

 tures are such as to indicate that they might be removed by in- 

 troducing a fourth term into the formulae, inversely proportional 

 to the cube of the absolute temperature ; but the trifling and 

 uncertain advantage to be thus obtained would be outweighed 

 by the inconvenience in calculation, and especially by the neces- 

 sity for solving a cubic equation in computing the temperature 

 from the pressure ; whereas with formulae of three terms, it is 

 only necessary to extract a square root, as the formula No. 2 

 shows. 



11. Although, for the mere determination of the maximum 

 pressure of a vapour at a given temperature, or its temperature 

 at a given pressure, a table, or a curve drawn on a diagram may 

 be sufficient, still there are many questions of thermo-dynamics 

 respecting vapours for the solution of which a formula is essential. 



Amongst these is the computation of the latent heat of evapo- 

 ration, which is equivalent to the potential energy or work exerted 

 by the vapour in overcoming external pressure, added to that 

 exerted in overcoming molecular attraction. For unity of weight 

 of a -given substance, this is a function of the pressure, tempera- 

 ture, and density ; but for a quantity of the substance such that 

 its volume when evaporated exceeds its volume in the liquid or 

 solid state by unity of cubic space, the latent heat of evaporation 

 is simply the differential coefficient of the pressure with respect 

 to the hyperbolic logarithm of the absolute temperature, as shown 

 in the formula No. 3; so that, although the densities of the 

 vapours of the seven fluids referred to in this paper are yet known 

 by conjecture only, and not by direct experiment, we can, from 

 the relation between the pressure and the temperature, determine 

 accurately how much heat must be expended in the evaporation 

 of so much of each of them as is necessary in order to propel a 

 piston through a given space under a given constant pressure. 



