82 



THE NATURE AND 



[SBCT. II. 



These are the principal researches that have been made on the force of vapours 

 at different temperatures, when in contact with liquids ; but, in order to render 

 the subject more complete, we must consider the force when not in contact with 

 the liquids which generate them, and also their density and volume. 



OF THE ELASTIC FORCE OF VAPOUR SEPARATED FROM THE LIQUIDS FROM 

 WHICH THEY WERE GENERATED. 



118. It has been remarked, that the elastic force of steam or vapour produced 

 by increase of temperature ceases to follow the same law where it is not in 

 contact with the liquid from which it was formed. (Art. 87.) The density of the 

 steam no longer increases, the force being solely that which prevents it expanding, 

 and is measured from the quantity it would expand if unconfined. The expansion 

 by the same increase of temperature having been found to be the same in all 

 gases and vapours, and the density as the compressing force, as far at least as 

 60 atmospheres, it becomes an easy task to compute this species of force within 

 that range of compression. 



This will also be further useful in determining the volume occupied by steam 

 of a given density and temperature as far as about 60 atmospheres : higher we 

 need not attempt to go for useful purposes ; and if we did, our rules would fail, for 

 there is not even a probable chance of the law, of the density being as the force, 

 extending to very high degrees of compression. 



119. The quantity a gas or vapour expands under a constant pressure, is 

 found by the following rule. 



RULE. To each of the temperatures before and after expansion, add 459. Then 

 divide the greater sum by the less, and the quotient multiplied by the volume at 

 the lower temperature will give the volume at the higher temperature. 



Or let t be the temperature with the volume v, and t' any other temperature, 

 then 



