( 251 ) 



on the immersion in water is mily ol)serveil in a sliglit degree '). 

 It must therefore be inferred that but few cavities are formed during 

 the dehydrations on the brancli — 0„. Tliis phenomenon is how- 

 ever much better noticed in the gel II (see below). The final cuvwc 

 of the rehydration Z f again has the points Oj and Oo, wiiicli 

 appear at the redehydration of water as in the case of silicic acid. 

 For the rehydration curve Z I does not converge, neither with 

 Z 1 nor witli A ],. It is true tliat the difference for the branches 

 O3 0.1 and Oi Oq is slight, but just as with silicic acid, the branch 

 O2 is vastly divergent from Oj Oo. The same hysteresis-phenomenon 

 appears. Line Z f, especially the brancli Oi 0^ can only bo realized 

 in the direction | (rehydration), the line Oi in the direction 

 I (dehydration). An intensified power of absorption is created on 

 Z I, that leaves its influence on Z ],. At the redehydration (Z [) 

 a point appears, showing the Siinie phüuomena as in silicic acid. 

 Starting from this point Oi the rcpiilibrium is obtained very slowlv 

 on the branch OOj; a weakening of the power of absorption takes 

 place, that leaves its influence behind, so that at the rehydration 

 it is not the branch Oj that is run back, but the branch Oi Oj. 

 As a result, here as with silicic acid return-curves may be realized 

 within the figure OOi OoO, if rehydration or redehydration 

 takes place, starting from a point on the line 0^ or on the line 

 Oi On. Two of these intermediate curves, one of the dehydration 

 I and one of the rehydration |, are indicated in the figure liy 

 dotted lines. 



Modifjciitinns. The modification caused by time, by which tlie 

 power of absorption is weakened, is also the same as with silicic 

 acid. The figure shows this for a Gel, that has grown 10 3-ears 

 older (curve A \ of III). The proportions are lower again than 

 were found before, when the same substance was seven years older 

 tlKin at the first determination. The fact of tiie Gel having been 

 under water for seven years has also weakened the power of ab- 

 sorption (curve A ], of II). Fur the sake of clearness the curves 

 Z \ and Z \ have been omitted. They differ very little from the 

 A [ line, and only diverge from it at higher vapour-tensions (at 

 about 10 — -11 num.), as is shown in the following table: 



') In silicic acid mniiy cavities are formed, wliicdi coiidensafe a considcraMp vo- 

 hmie of air. Yide Verslag Kou. Akad. of April ii, 1898, p. 498—500. 



