534 BESREL ON BAROMETRICAL 



finitely small. Whether all gases have a maximum of density 

 dependent on temperature (as is known to be the case with car- 

 bonic acid gas), so that they only differ from vapours by the 

 amount of the maximum (or specifically), cannot at present be 

 decided, and is not here touched on. 



So long as vapours have a less density than the greatest 

 they can attain in the respective temperatures, they are not 

 physically different fi-om gases; they follow Mariotte's law; and 

 Gay Lussac has shown that they possess the same expansibility 

 by temperature which is common to all gases. So long, therefore, 

 as they have not attained the maximum of their density, they 

 comport themselves, whether alone or mixed with gases, pre- 

 cisely as gases do. A pressxire does not produce a change of 

 state in them any more than in gases : that change first takes 

 place, equally whether they are mixed or unmixed, whenever an 

 attempt is made to cause their density to exceed its maximum. 

 This can be done by lessening the space in which they are con- 

 tained, in which case the gases, if present, remain unchanged in 

 consequence of their unlimited compressibility. If, further, a 

 space is filled with a gas which exerts a pressure p upon an unit 

 of surface, the introduction of another gas, which if alone would 

 exert the pressure p^ on the same unit, produces no other pliy- 

 sical consequence than that this unit now sustains the pressure 

 p ■\- Pi', but it would sustain precisely the same pressure, if, in- 

 stead of the second gas, a vapour were introduced exerting when 

 alone the pressure p^. Lastly, different kinds of gases mix with 

 each other, as well as with vapour, in any arbitrary propor- 

 tions. 



There is therefore throughout, no difference between the phy- 

 sical comportment of a mixture of two gases, and of a gas and. 

 vapour ; consequently the circumstances of the second mixture 

 can teach us nothing which we might not learn from those of the 

 first. The comportment of the mixtui*e of air and aqueous va- 

 pour, which Dalton's experiments have fully manifested, is not, i 

 therefore, more instructive than that of any mixture of two 

 gases ; and a theory which could not be constructed upon thei 

 latter, cannot find support in the former. It could not, thei'e- 

 fore, have been deduced from the comportment of a mixture of 

 vapour and air that the air does not press the vapour, unlessj! 

 for tiie presupposition that pressure changes vapour into fluid ;f 

 for this presupposition, however, there is no justifying fact. ! 



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