34 THE CHEMISTRY AND PHYSICS OF THE CELL 



It must also be mentioned that the solvent is probably an important factor 

 in determining the colloidal or non-colloidal nature of a substance; e. g., soaps form 

 true solutions in alcohol and colloidal solutions in water; gelatin forms colloidal 

 solutions in water but not in ether, whereas rubber forms colloidal solutions in 

 ether but not in water. 



Closely related to solubility is the phenomenon of imbibition, which may be 

 defined as the taking up of a fluid by a solid body wathout chemical change. Not 

 all colloids possess this property, but it is shown by most of the organic colloids, 

 particularly the proteins. Fick distinguishes capillary, osmotic, and molecular 

 imbibition, the latter of which is the form exhibited by colloids, and it occurs in- 

 dependent of the existence of pores or other preformed spaces in the imbibing body. 

 The imbibition of water by colloids is more than a simple mechanical process, for 

 it is accompanied by a contraction in the total volume of solid and water, and by 

 the evolution of heat. The forces developed are far greater than those of osmotic 

 pressure; e. g., to prevent imbil>ition of water by starcli requires a pressure of over 

 2500 atmospheres. On the otlier hand, the physical properties of an aqueous 

 colloidal solution show that the colloid is not chemically combined in the form of a 

 hydrate. To describe this peculiar relation Hofmeister and Oswald recommend 

 the term "mechanical affinity." Hardy has shown that water held in a gelatin 

 jelly cannot be removed by great pressures (400 pounds to the square inch), but 

 after the nature of the jelly is so changed by formalin that it is no longer liquefiable 

 by heat, the water can be easily expressed from the loose meshwork that is formed. 

 It would seem from this that the imbibition and retention of water by colloids may 

 be closely related to surface phenomena. Hofmeister has shown that organized 

 animal tissues obey the same laws of imbibition as do simple gelatin plates, and 

 probably this phenomenon of colloids is very important in physiological and patho- 

 logical processes. 



Non-diffusibility. — The lack of power to pass through animal and parchment 

 membranes, which was Graham's starting-point in the study of colloids, is also 

 only a relative condition. This is shown by the following figures, giving the rela- 

 tive time required by the same amount of different substances to pass through a 

 certain diffusion membrane: 



Sodium chloride 2 . 33 



Sugar 7.00 



Magnesium sulphate 7 . 00 



Protein 49.00 



Caramel 98.00 



This difference of time is so great, however, as to permit of separation of salts 

 from proteins, etc., by dialysis, a process in constant use. Primarily the ability 

 to diffuse through a given membrane requires that the diffusing substance be 

 soluble in the membrane. Diffusion menil)ranes are always composed of colloids, 

 e. g., animal bladders, or parchment, whicli is a colloidal cellulose. Crystalloids 

 are generally solul>le in colloids, while colloids are little or not at all soluble in 

 other colloids, and hence do not dilTuse through one another readily and permeate 

 diffusion membranes very slowly. For example, if a stick of agar jelly be jilaced 

 in a solution of ammoniated copper sulphate (a crystalloid), and another be placed 

 in a solution of Prussian blue (a colloid), it will be found that the copper solution 

 penetrates the agar completely before the colloidal solution of Pru.ssian blue has 

 penetrated it at all. This property is of great importance, undoubtedly, in keep- 

 ing different colloidal constituents of the cell in given localities witliin its proto- 

 plasm, c. g., tl)e colloidal glycogen remains wliere it is formcil in tlie cytoplasm, 

 unaljle to escape from tlie cell, whereas the crystalloidal sugar from which it is 

 formed and into which it is converted, diffuses raiiitlly into or out of the cell. 

 The osmotic pressure of tiie colloids is extremely small. The closely related 

 phencjinena of dijliision, (U'prc.ss'ion of freezing-point, and ehrnlion of boiling-point, 

 are also exhibited by colloids to l)ut an extremely slight degree. For example, in 

 one experiment, tlie di.s.solving of from 14 per cent, to 44 ]K'r cent, of egg-albumin 

 in water lowered the freezing-point but 0.02° to 0.00°; and some other colloids 

 have even less effect. The results of the latest and best experiments seem to in- 

 dicate tiiat the trifling ericcts of colloids upon osmotic pressure and upon freezing- 

 and l)oiling-i)oin1s oiiserved in colloidal solulions are due to the colloids tliemselves, 

 ratlier than to included inii)urities, although it may possibly be th.at some oi 



