278 M. V. Regnault on the Elastic Forces of Vapours 



of the boiling liquid, and especially upon the briskness of the 

 ebullition. 



The observations which 1 have just described confirm the fact 

 announced by Rudberg, but it appears to me that they also show 

 its cause ; for whenever the thermometer only indicates the 

 temperature under which pure watery vapour is in equilibrium 

 with the external pressure, we find that the reservoir is wet. 

 On the other hand, the instrument shows a higher temperature 

 when its reservoir is dry, a condition which I have only been 

 able to realize in the strata of vapour immediately above the 

 super-heated liquid. 



I think, therefore, that the vapour, originating in boiling 

 saline solutions, is equal in temperature to them, and does not 

 possess an elastic force much superior to the hydrostatic pressure 

 which they exert upon it. If the temperature of this vapour be 

 suddenly lowered to the degree which corresponds with its satu- 

 ration under this pressure, this arises from the circumstance 

 that in consequence of the small capacity for heat possessed by 

 vapours in comparison to their volume, the excess of heat is 

 rapidly absorbed by external refrigerating causes, and especially 

 by the vaporization which acts upon the infinity of minute liquid 

 globules, which are constantly projected into the atmosphere of 

 vapour, at the moment when the bubbles burst at the surface of 

 the boiling liquid. 



I have determined, with several aqueous solutions, the tempe- 

 rature to which they must be raised in a manometric apparatus, in 

 order that the vapour, thus produced in vacuo, should be in equi- 

 librium with a pressure of 760 millimeters. The excess of this 

 temperature over that of 100° C. which would give water vapour 

 this tension of 760 millimeters, if it were in presence of pure 

 water, may seem, as M. Pliicker has lately pointed out, to mea- 

 sure the excess of afiinity possessed by watery vapour for the 

 saline substance, in comparison with that which it presents for 

 the similar particles of water. But in order that this afiinity, 

 thus measured, may constitute a specific character of the sub- 

 stances, it is necessary that it should vary in proportion to the 

 quantity of the same salt in the solution. Now I have found 

 that this is not the case ; the variation follows a more complex 

 law, which appears to depend upon the nature of the salt. 



I attach particular interest to the comparison of the tempe- 

 rature at which the vapour given off^ in vacuo by a saline solution 

 is under a pressure of 760 miUimeters in equilibrium, with the 

 temperature presented by the same solution when boiling under 

 the same pressure. Unfortunately, it is nearly impossible to 

 determine, with any degree of precision, the boiling-point of a 

 concentrated saline solution. The ebullition is always irregular ; 



