54 



without regard for lack of grounds for the assumptions involved, 

 Several of these still survive because, while they cannot be proved, 

 they are not of a nature to be easUy disproved. It wiU. probably be 

 sufficient to outline two of the more recent ones. 



Perhaps it would be unfair to consider Fischer's theory of imbibition 

 as attempting to give an explanation of the molecular mechanism 

 of the phenomenon. He regards gelatin as a substance capable of 

 existing in different degrees of association or polymerisation ; in other 

 words, the particles of gelatin may vary greatly in size, dependent 

 upon conditions to which they are subjected. Thus rise of tempera- 

 ture or increase in concentration of acid or alkali, causes the particles 

 to become smaller in size, the change being reversible. The particles 

 are assumed to be capable of becoming most heavily hydrated, that 

 is, of absorbing most water, when they have a medium diameter. 

 The particles of neutral gelatin are large and capable of absorbing 

 comparatively little water. Increasing the concentration of acid 

 decreases the size of the particles, making them capable of absorbing 

 more water, and the gelatin swells until the size of particle most 

 readUy hydrated is reached. As the particles become still smaller, 

 the swelling becomes 'less untU finally the particles become so small 

 that the gelatin apparently goes into solution. 



The theory of Tolman and Steam assumes that, because of their 

 amphoteric nature, protein colloids have marked tendencies to 

 adsorb hydrion from acid solutions and hydroxidion from alkaline 

 ones. In a solution of a strong acid, the adsorbed hydrogen ions, 

 together with a corresponding number of anions, form a " double 

 layer " on the walls of the pockets or pores in the interior of the gel, 

 and this leads to swelling and imbibition of water by electrostatic 

 repulsion. The addition of neutral salt or excess of strong acid to 

 such a swollen colloid will furnish ions in the interior of the pockets 

 which will tend to arrange themselves so as to neutraUse the electrical 

 fields of the adsorbed layer and thus being about a reduction of the 

 swelling. The addition of a neutral salt to the acid solution tends to 

 neutralise the electrical field of the adsorbed acid, making it easier 

 for more acid to get to the surface of the pockets, thus leading to 

 increased adsorption. Polyvalent ions are more effective than 

 imivalent ions in reducing swelling because, while taking up no more 

 room than univalent ions, they are twice as effective in neutrahsing 

 an existing electrical field. 



In contrast to these stands the theory originated by Procter and 

 developed by him in collaboration with his pupils. By very extensive 

 investigations Avith gelatin and aqueous solutions of acids and salts, 

 he succeeded in finding quantitative relationships between several of 

 the variable factors involved. Once a foothold was gained in the form 

 of an equation, it was found possible to make big advances merely 

 by an apphcation of mathematics. Procter built up his theory from 

 experimental data; more recently J. A. and W. H. Wilson worked 

 in the opposite direction by purely mathematical reasoning from the 

 assumption of the existence of a certain hypothetical substance and 

 calculated what results Procter should have found experimentally. 



