C. Barns — Compressibility of Colloids. 



291 



7. As has been stated, these inferences refer specifically to 

 liquid colloidal solutions. The interesting question now pre- 

 sents itself : In what respect does compressibility change when 

 the liquid colloid thoroughly coagulates,* or changes in relation 

 to viscosity from a viscous liquid to a soft solid. 



The results for coagulated gelatine solutions are given in 

 Table 5 ; but the datum j3 is here, at the lower temperature, 

 merely a superior limit compressibility, for reasons which I 

 shall presently explain. § 8. The true compressibility is 

 probably less than -J /3 ; indeed no true compressibility may 

 have been measured. As temperature rises (33°, 41°, 51°), 

 approaching the melting point, the normal compressibility)- of 

 the solvent water is rapidly approached here, as was the case 

 in Table 2 for albumen. Stress therefore breaks down the 

 friable solid, triturates it as it were, the colloid being altogether 

 too soft to resist the advancing column of mercury appreciably 

 like a solid. Whenever a sufficient number of internal discon- 

 tinuities is at hand (unstable configurations, virtually) the solid 

 is proportionately liquid. 







Table 



5. — Coagulated colloids. 



Gelatine 10 



per cent. 





Albumen* coagulated at 100°. 



e, l 



P 



l/L 



0x 10 6 



6, L 



22° 







.000 



— 



22° No motion of me- 



20-2 cm 



97 



1 



10 



12*7 cm niscus observed as far 





210 



2 



10 



as200 atm . At higher 













— 



pressures threads of 



Another 



sample : 







mercury pierce the 



24° 







•0000 



• 



core and make further 



13-2 cm 



105 



23 



26 



measurement impossi- 

 ble. 



32-1° 







•0000 



— 





x 3.3cm 



104 



49 



47 



* Heated about 10 minutes, the 











white thread expands. Examined 



40*8 







•0000 



— 



next day after setting. Meniscus 



13-3 cm 



92 



38 



41 



fouled by sulphur corrosion. 



51° 







•0000 



— 





x 3.4cm 



117 



55 



47 





8. The compression of the solid colloid is accompanied by 

 characteristic phenomena (as has just been intimated), deserv- 

 ing special examination. They are shown in the annexed 

 figures, where the contiguous threads of mercury (below) and 

 colloid (above) meeting at the lower meniscus are alone repre- 

 sented, the walls of the capillary tube containing the threads 

 being ignored. 



* That time for setting is essential here has already been emphasized by de 

 Metz, Fraas and others (1. a), and in the above work with albumen, 

 f Cf. § 11, relative to suggestions of Carus- Wilson. 



