276 M. Regnault on the Elastic Force of Vapours. 



2. Where the liquid does not possess a great molecular cohe- 

 sion; for otherwise the liquid boils in an intermittent manner 

 with violent j urn pings, and the determinations by the dynamic 

 method become very uncertain. 



Both methods have been successfully applied to mo3t of the 

 volatile liquids taken for experiment, and I have been able to 

 determine their tensions from the lowest temperatures to those 

 which correspond to pressures of 12 to 15 atmospheres. Most 

 liquefiable gases give liquids having a great molecular cohesion, 

 and which boil with great difficulty in spite of their extreme mo- 

 bility. Their tensions can only be determined with certainty by 

 the statical method. In order to apply the dynamic method — 

 that of ebullition — the thermometer can only be placed in the 

 vapour of a boiling liquid when its boiling-point is above that of 

 the surrounding atmosphere, for otherwise the vapour might 

 become superheated, and the indications of the thermometer 

 would be faulty. If the thermometer dips in the boiling liquid, 

 it exhibits variations of temperature during ebullition, although 

 the pressure remains the same. The indications of the thermo- 

 meter often change according to the manner in which the heat 

 is applied. The ebullition is not continuous; violent jumpings 

 take place accompanied by a sharp sound like that emitted by 

 inverting the water-hammer. These effects greatly vary with 

 the pressure. In certain liquids they take place at pressures 

 below that of the atmosphere. In others they only take place 

 under high pressure. 



In this limited space I must omit any mention of the obser- 

 vations I have made on each substance, of the graphic method of 

 constructing the results, and of the interpolation formula by 

 which I have endeavoured to represent my researches. I will 

 only say that, of all the methods of interpolation tried, the ex- 

 ponential formula proposed by Prony, and applied by Biot to 

 aqueous vapour under the form 



log F=« +&*'+<*', 

 applies best to all the substances I have tried. This formula 

 has the advantage of containing five constants, for the de- 

 termination of which, five points of the curve may be chosen 

 having equidistant abscissae, so that the curve represented by 

 the formula can deviate in the intermediate parts very little 

 from the graphic curve. For a great number of the substances 

 which I have studied, by a convenient arrangement of the fixed 

 points which serve to calculate the constants, and without de- 

 viating too much from the observed data, a formula with two 

 exponentials may be calculated, 



logF=a + fot' + c£', 

 in which the term c& only introduces values which are less than 



