100 



equilibria; one oughi to state the value whicli is attained in liquid 

 water ^). 



The explanation of the phenomenon becomes somewhat difTei-ent, 

 if we do not let the gelatine (oelloidin) swell in pure water (alcohol) 

 but in solutions. In case of the dissolved substance being volatile 

 (example : celloidin-alcohol-water), the circumstances are exactly the 

 same as above mentioned ; but, if the substance is e.g. a salt, the 

 thing changes. Von Schkop^der has made some experiments about 

 this question, and he ascertains that \\„„„„o-norm. sulphate solution 

 alread}^ lowers the vapourtension of gelatine so much, that there is 

 not even a decrease in vapour, but an increase. After the results 

 described above, von Schroeder's argument loses all weight, and 

 his result is sure to be due to chance. 



It is a fact we have repeatedly observed, that gelatine (celloidin) 

 which had swollen in the vai)Our of a salt solution (NaCl in water, 

 resp HgCl^ in alcohol) absorbs much more still, when brought into 

 the liquid; J Gr. celloidin e.g. gains 1,77 Gr. in weight in a solution 

 of 4 7„ snblimate in alcohol of 96"/,,, whereas only 0.89 Gr. is 

 absorbed in the vapour of alcohol, and consequently still less in 

 the vapour of a solution in alcohol. The ditFerence betweeji the 

 swelling in a pure Vu\i\'u\ and in a solution liiuls its cause in a 

 substance, when in the vapour of a solution, never being able to absorb 

 anything but the solvent, as long as the dissolved substance is not volatile; 

 when placed in the liquid itself, it also absorbs liie dissolved substance. 



This is a w^ellknown fact about gelatine; we have ascertained 

 by the experiments with celloidin mentioned above, that this substance 

 too had not only absorbed alcohol, but also sublimate. So in these 

 cases it is clear for other reasons, that a substance swollen in vapour, 

 when brought into the liquid phase, must still absorb more, and 

 that the state attained in vapour is not a true equilibrium '). In 

 vapour e.g. celloidin passes into celloidin -|- alcohol, until the vapour- 

 tension of this phase has become equal to that of the liquid phase: 

 alcohol + HgCl.,. Now if one brings the celloidin jihase under the 

 liquid, the HgCl, ditfuses into the celloidin; diminishes so to say 

 the vapourtension of the alcohol which is contained therein, and 

 consequently a further absorption of alcohol must take place. If we 



1) SU-ictly speaking one must not speak, of pure water, but of a saturated solution 

 of gelatine, agar etc. in water; of course, ihe difference really is exceedingly minute. 



2) Cf. Tolman's views, J. Amer. Chem. Soc. 35, 307 (1913). We have 

 "assumed with Tolman that every tubslance evaporates somewhat, no matter how 



little; if one objects to this, one must speak of an equilibrium which hes differently 

 in consequence of passive resistances (Gibbs). 



