558 



presents the same course as that found bj me for carbon ; the initial 

 part as the curve for a heat of mixing, the almost rectilinear middle 

 portion, the end in a curve with the concavity downward. Unfor- 

 tunately we have no reason to believe that the silicic acid examined 

 by Bei.i.ati and Finazzi possesses exactly the same constants as that 

 on which Van Bkmmei.en and Andkkson performed their determina- 

 tions of the vapour tension, as the properties greatly depend on the 

 preparation. This is, however, the case in the experiments with car- 

 bon, de,scribed above. 



In the absorption of water va|»our by carbon we have, therefore, 

 to do with a system of which the isotherm and the curve of the 

 heats of sorption are in perfect agreement with the same curves of 

 those silicic acid gels that present a so-called "turn". 



In silicic acid it is very probable that in the tlat piece very fine 

 capillaries ai'e getting filled with water, for absorption of water 

 causes the opaque substance to become transparent again. Zsigmondy 

 and Anderson ') pointed out that the radius of these fine capillaries 

 can be calculated from the vapour tension of the water in tlie flat 

 piece; they then arrived at values of the order of magnitude ] .3 

 10'^ mm. for the initial part, and 2.6X^0"^ mm. of the end of 

 the flat piece. And they showed further that when tlie same silicic 

 acid gel is changed into an alcohol or benzene gel, and the radius 

 of the capillary is calculated from the vapour tension of the alcohol 

 or the benzene, values are obtained for this radius of the same mag- 

 nitude as in water. This pleads very strongly in favour of the view 

 that the flat middle piece is due to the filling of capillaries, which 

 gradually become slightly wider, hence on micro-porosity. 



Patrick') repeated these experiments with liquid carbonic acid 

 and liquid sulphur dioxide with silicic acid gel. Then he found, 

 however, much less concordant values for the size of the capillaries; 

 he tried to explain this by the greater thickness of the capillary 

 layer near the critical point. 



Bachmann'), working in Zsigmondy's laboratory, also explained 

 the flat middle piece in the isotherm of carbon and water by a 

 system of such fine capillaries. The substance being opaque, it can- 

 not be ascertained if this property becomes stronger in the middle 

 piece. 



1) Zeitschr. f. pliysiltal. Chemie, 88, p. 191 (1914); Zsigmondy, Lehrbuch der 

 Kolloidchemie, 4th edition, p. 219—234. 

 ') Patrick, Diss. Göttingen, 1914. 

 ') Bachmann, loc. cit. 



