2 Barns — Thermodynamic Relations of Hydrated Glass. 



from which it differs in refraction and density. Capillary 

 tubes at the end of the process have become glass rods. The 

 original thread of water is replaced by a solid core of water- 

 glass more than twice the original diameter.* 



Nevertheless all capillary tubes with a solid core of this kind 

 break throughout their length in the lapse of time, however 

 carefully cooled. Cf. § 5. 



3. An explanation of these phenomena may be given in the 

 terms of the compressibility observed along the successive 

 isotherms for the different concentrations of glass solution at 

 210°. This at first shows a relatively small value, implying a 

 steep isotherm in a Clapeyron (pv) diagram. Thereafter com- 

 pressibility passes through a relatively enormous value (over 5 

 times the initial result), implying a nearly horizontal isotherm. 

 The reaction ends with even smaller compressibility than the 

 first observed, implying steeper isotherms than the initial curves. 

 Now although these isotherms are all at the same absolute tem- 

 perature, the water-glass is becoming continually more concen- 

 trated and viscous. Hence the corresponding temperatures of 

 the isotherms in van der Waals' sense are continually decreasing. 

 It follows that the reaction considered as a thermodynamic 

 process is a march through the critical region of certain phases 

 of the water-glass examined. 



4. In the light given by J. W. Gribbs's famous investiga- 

 tions, one may arrive at clear notions by adopting but two 

 phases of the water-glass, for comparison. Many phases may 

 coexist ; two are selected in the interest of brevity and called 

 phase 1 and 2, respectively. During the earlier stages of the 

 reaction (dilute water-glass) phase 1 is stable. At the end of 

 the reaction (concentrated water-glass, subsidence of volume 

 contraction) phase 2 is stable. Both cases correspond to steep 

 isotherms. Toward the middle region of the reaction (max- 

 ima of compressibility) phases 1 and 2 are mutually stable in 

 presence of each other. Hence the horizontal isotherms 

 corresponding to the critical region are cut through in a march 

 from greater to smaller corresponding temperatures. 



That phase 2 is really unstable during the first stage of 

 reaction is shown by the approximate constancy of the values 

 of compressibility, throughout intervals of pressure as high as 

 could be applied. For instance, at 185°, 



Interval, p= 20-100 100-200 200-300 300-400 atm. 



10 6 X£= 146 144 142 146 



And fifteen minutes later 



10 6 X/3= 188 176 201 189. 



* Diameters of threads of water and of water-glass respectively in tube No. 4 r 

 •024 and -071^; in tube No. 5, -033 and -059 cm ; in tube No. 6, -053 and 125 cm - 

 etc. ' r 



