532 Mr. W. J. M. Rankine on Formula for the 



the table, are suited for millimetres of mercury as the measure 

 of pressures, and for the scale of the Centigrade thermometer ; 

 but logarithms are given, by adding which to them they can be 

 easily adapted to' other scales. 



The limited time which has elapsed since the pubUcation of 

 M. Rcgnault's experiments prevents my being yet able to bring 

 the details of the investigation, of the formulae, and of the com- 

 parison of their results with those of experiment, into a shape 

 suited for publication ; but I shall here add some remarks on 

 their degree of accuracy and the extent of their applicability. 



8. M. Regnault explains, that his experiments were made by 

 two methods; at low temperatures, by determining the pressure of 

 the vapour in vacuo ; at high temperatures, by determining the 

 boiling-point under the pressure of an artificial atmosphere. For 

 each fluid, the pressures determined by both those methods were 

 compared throughout a certain series of intermediate tempe- 

 ratures. 



For all fluids in a state of absolute purity, the results of those 

 two methods agreed exactly (as M. Regnault had previously 

 shown to be the case for water) . 



The presence, however, of a very minute quantity of a foreign 

 substance in the liquid under experiment was sufiicient to make 

 the pressure of the vapour in vacuo considerably greater than 

 the pressure of ebullition at a given temperature ; and it would 

 appear, also, that a slight degree of impurity affects the accuracy 

 even of the latter method of observation, although by far the 

 more accurate of the two when they disagree. 



9. The degree of precision with which it has been found pos- 

 sible to represent the results of the experiments by means of the 

 formulae, corresponds in a remarkable manner with the degree 

 of purity in which, according to M. Regnault, the liquid can be 

 obtained. 



Sulphur et ofCarhon, M. Regnault states, can easily be obtained 

 perfectly pure. For this fluid, the agreement of the pressures 

 computed by the formula with those determined by experiment 

 throughout the whole range of temper^re from —16° Cent, to 

 4-136°, is almost as close as in the case of steam. 



JEther and Alcohol are less easy to be obtained perfectly pure. 

 The discrepancies between calculation and experiment in these 

 cases, though still small, are greater than for sulphuret of carbon. 



For aether the formula may be considered as practically correct 

 throughout the whole range of the experiments, from —20° 

 Centigrade to +116°; but for alcohol below 0° Centigrade, the 

 discrepancies, though absolutely small quantities, are large rela- 

 tively to the entire pressures ; and the formula can be considered 

 applicable above this temperature only. 



