272 M. V. Regnault on the Elastid Forces of Vapours 



artificial atmosphere. The former method has been followed 

 for the low temperatures; the second has been exclusively 

 employed in the high temperatures. In all cases they are 

 arranged in such a manner that the curves of the clastic 

 forces given by the two methods should present a common part 

 by which we may judge of their coincidence. I have already 

 shown, in my memoir on the elastic force of water vapour, that 

 this coincidence is exact in the case of water, the two methods 

 giving perfectly identical results. I have ascertained that the 

 same is the case with other volatile liquids, provided they are in 

 a state of perfect purity. When a liquid contains even an ex- 

 tremely small portion of another volatile substance, the two 

 methods give different values for the elastic force of its vapour 

 at the same temperature ; so that this furnishes an extremely 

 delicate means of judging of the homogeneity of a volatile body. 



Sulphuret of carbon is easily obtained in a state of purity, 

 but the same cannot be said of alcohol and aether. As to chlo- 

 roform, whatever care may be taken in its preparation, it always 

 contains several intermixed substances, which it is impossible to 

 separate by fractional distillation even when operating upon large 

 masses. The vapour has a different elastic force and a sensibly 

 different density, according as the first or the last products of 

 distillation are operated with. Chloroform has also constantly 

 given different values for its elastic force at the same temperature, 

 according to the method by which it was determined. This cir- 

 cumstance is easily seen from the preceding table, in which 1 

 have only given a single series of the experiments which I have 

 made upon chloroform. 



Some liquids become modified in their molecular constitution 

 when boiled for a long time under high pressures. It thus often 

 happens that at the end of a series of experiments the liquid is 

 found not to have the same boiling-point under the ordinary 

 pressure of the atmosphere as at the commencement. Essence 

 of turpentine offers a remarkable example of this. Thus, a con- 

 siderable quantity of the essence (30 or 40 litres) having been 

 boiled for several hours under a pressure of 7 to 8 atmospheres, 

 became almost completely converted into a liquid which boiled 

 at a temperature above 230° C. under the ordinary atmospheric 

 pressure. I put this modified liquid aside in order to deter- 

 mine its nature, but it has since been thrown away by mistake. 



Other liquids appeared to undergo molecular modifications, 

 rendered sensible by the tension of their vapours, when left by 

 themselves for a longtime in hermetically closed tubes. iEther 

 has presented me with a curious example of this. I shall return 

 to this subject on another occasion. 



I may remark, in conclusion, that the method of ebullition in 



