( 250 ) 



made use of the same sions (sec lii;-.) viz. A | tor the curve of' 

 dehvdnition of water, / | fur that of tlie reliyilration of water 

 and Z I for tliat of the rcdehydrafion of water. Tlie eqiiilil)riiim 

 at a certain vapour-tension was only gradually approximated on 

 A I, tlie more slowly in proportion to the latter's differing less 

 from the tension of saturated vapour (12.7 m.m.). At 122m.m. aloss 

 in 1 mouth's time was still noticeable after 5 months; at 9 m.m. 

 the loss in 1 month's time was no longer noticeable after 3 months ; 

 and at 0° m.m. after one week. On the contrary the equilibrium 

 in Z I and Z | was obtained within 1 — 2 days, except on part of 

 Z I (from point to ()j) where the phenomenon of Hysteresis 

 showed itself again. 



From the figure we see that the curves agree with tiiose for the 

 Cu 0- and the Si Oo-hydrogel. 



The proportion of water at 15" is greater tlian that of these two 

 (I only give here that at 12' and at in.ni. vapour-tension). 



CuO 

 Si Oo 



but is equally continuously (h:'peni]ent on the vapour tension. 



Point 0, which shows a break in the continuity in silicic acid (in 

 consequence of a transformation in the gel and the diminution of 

 the power of absorption resulting therefrom) and at the same 

 time the turnpoint of the process of de-hydration, this point is not 

 so clearly seen in the hydrogel of Fcj 0.;. But it betrays itself in 

 the following manner. If tlio gel, arriving in any point of the line 

 A I, is provided again with water from this point (by the exposition 

 to a higher vapour-tension up to 12' m.m.) it only absorbs ± 

 0,2 lioO ; comp. for this on fig. 1 the lines A |, starting from the 

 points a, b, c, d. This rule however only holds good if the propor- 

 tion of ± 3,5 IIjO is not yet reached; beiow this the gel absorbs in 

 case of rehydration so much water, that the proportion in tension 

 of saturated vapour amounts to 3r 3,G"'. It is therefore about here 

 that the possibility of reversing the further process of the dehydra- 

 tions begins, and there is a point which answers in that respect 

 to the turning-point in silicic acid. At the end of the dehydration 

 in point Oq (vapour-tension == 0" m.m.) the appearence of air-bubbles 



