172 Condensation of Carbonic Acid on Smooth Glass Surfaces. 



that in reality the layer can never increase beyond that limit 

 in which the capillary pressure, diminishing with the distance, 

 becomes equal to the vapour-tension of liquid carbonic acid, 

 and that outside this limit an atmosphere of carbonic acid 

 of diminishing density must be present. These considera- 

 tions, it is true, explain the condensation observed, but not 

 its extended continuance for years. Moreover theory requires, 

 in agreement with experience, an instant adjustment of equi- 

 librium in gases under the influence of pressure. The long 

 duration of the condensation is then scarcely comprehensible 

 otherwise than on the assumption that the glass is not per- 

 fectly impenetrable, and that the particles of liquid carbonic 

 acid on being pressed into the molecular interstices of the 

 glass have to overcome a resistance which, as the observed 

 course of the condensation shows, rises in a proportion increas- 

 ing with the time. 



Only by further prosecution of the experiments can it be 

 decided whether a point of time can be found when the 

 forcing of the fluid carbonic acid into the glass is indefi- 

 nitely small. If such a point is attainable and attained, 

 then must the processes of condensation become essentially 

 different. The thickness of the layer of liquid carbonic acid 

 would then no more continually diminish by its being forced 

 into the glass, nor would the overlaying gaseous stratum of 

 carbonic acid undergo simultaneous diminution. With the 

 increasing thickness of the fluid layer in course of conden- 

 sation, that of the atmosphere of carbonic-acid gas would con- 

 tinually diminish, and hence also the pressure which this 

 atmosphere exerts on the surface of the fluid, until a surface- 

 tension is reached equal to that exerted by the carbonic-acid 

 vapour at the prevailing temperature. The moment this point 

 is reached, as long as the conditions remain the same, no 

 further condensation or vaporization of the liquid layer of 

 carbonic acid can take place; but it must set in again directly 

 there is an alteration of pressure or temperature. When a 

 rise of temperature ensues, gas will escape from the fluid 

 layer; and as soon as a state of equilibrium has been restored, 

 the diminution of thickness brought about in this way can be 

 calculated by measuring the volume of gas set free. 



Thus it appears, if not very probable, at least possible to 

 determine for a series of temperatures a series of differences 

 of thickness of the particular levels at which the carbonic acid 

 has come to rest, and to obtain, in terms of the vapour-tension 

 of carbonic acid corresponding to the temperature, a mea- 

 sure of the capillary attractions which were exerted at these 

 differences of thickness. How far this conclusion is allowable, 





