MEASUREMENT OF HEIGHTS. 537 



If no special hypothesis be made as to the molecular consti- 

 tution of gases and vapours, it is plain that a particle of gas 

 must press an adjoining particle of similar or dissimilar consti- 

 tution with equal force (i. e. with the same force with which it 

 endeavours to expand). Without a special hypothesis Dalton's 

 view contradicts the fundamental propositions of aerostatics. But 

 such a view cannot be maintained unconditionally until proof 

 is adduced that no supposition, such as is here referred to, is 

 mathematically possible. On the other hand, the view which I 

 have developed of the comportment of vapours, does not require 

 to be justified by a special hypothesis. We may regard, as the 

 immediate result of experiment, and as the distinguishing mark 

 between vapours and gases, that the density of vapours cannot 

 be increased beyond a certain degree dependent on temperature. 

 But if we desire to enter likewise on the molecular constitution, 

 it is easily conceivable that there may exist a distance between 

 the ultimate particles of vapours, in which their attractive force 

 is equal to the repulsive force arising from the temperature, so 

 that every decrease of distance renders the attractive force pre- 

 dominant, and consequently unites the particles. 



7. 



If, notwithstanding what is here said, I have followed Dal- 

 ton's view in Sect. 5, in respect to the dry constituents of the 

 air, I can the less omit to examine the deductions from it in re- 

 gard to the aqueous vapour. This examination must also be 

 pursued, if we desii-e to learn whether the observed distribution 

 of the aqueous vapour in the atmosphere can be made to tell 

 for or against Dalton's hypothesis. I will, therefore, assume 

 with Dalton the aqueous vapour in the atmosphere to be pressed 

 only by its own higher strata, or to form an atmosphere by the 

 equilibrium of its own parts alone. The change of the pressure 

 of the atmosphere of vapour, corresponding to the increase dx 

 of the elevation x, is according to formula (4.), 



"^P'- 336-905 \a + x) "^ "" ' 

 or, according to the notation subsequently introduced, 



It» density 8, until it reaches its maximum, follows Mariotte's 



