PRINCIPLES OF GENERAL PHYSIOLOGY 



Moreover, it is characteristic of matter in the colloidal state (see Chapter IV.) 

 not to be in permanent equilibrium it is what has been called a "non- 

 conservative system." It will become plain in later parts of this book how 

 large a part colloidal phenomena play in the life of the cell. Van Bemmelen 

 (1910, pp. 230-233) showed in 1896 that, if a preparation of colloidal silica 

 as a moist jelly be taken and exposed to air containing various percent.!-. - 

 of water vapour, the amount of water contained in the colloid varies continuously 

 with the tension of the aqueous vapour. But the point of importance in the 

 present connection is that, in certain regions of the curve, the amount of water 

 present in the colloid at a given tension of water vapour is not the same if 

 the silicic acid has previously been exposed to a lower tension, as it is if it lia^ 

 been exposed to a higher one. For example, if it has previously been in a 

 drier atmosphere, and is then placed in one with a tension of water vapour of 



(i-3 



B 







FHJ. 4. 



WATER CONTENT OF A SILICIC ACID OEL, IN EQUILIBRIUM 

 WITH DIFFERENT TENSIONS OF WATER VAPOUR. 



Ordinates tension of water vapour in millimetres of mercury. 



Abscissae water content of gel : A, when exposed to increasing tensions ; B, when 

 exposed to decreasing tensions. 



Showing "hysteresis." Inorganic systems have time factors and "life histories." Thus, 

 from the water content corresponding to a tension of i; mm. Hg (mid height of 

 figure), we have information as to whether the previous history has been one of 

 exposure to increasing or to decreasing tension of water vapour. 



(From van Bemmelen's fig. 12, 1910, p. 247.) 



6'3 mm. Hg, the water contained in the gel (A) (Fig. 4), after it has come 

 into equilibrium with the gas phase, is less than one half of what it is if 

 placed in the same atmosphere after previous exposure to one of a higher wain 

 vapour tension, say of 12'7 mm. Hg (B). Accordingly, if such a gel be placed in 

 a water vapour tension of 6'3 mm. or thereabouts, information can be obtained of 

 its previous history. The phenomenon here described is known as " hysteresis." 



Again, it has been held that an organism differs from non-living matter in that 

 its state at any moment depends not only on its previous history, but also on it- 

 future history. Here, also, similar conditions are not unknown in pure chemist rv. 

 The relative concentration of the components of a reversible reaction is determined 

 at any time, not only by the initial state, but also by the final state, namely, 

 that of equilibrium. The rate at which acetic acid and methyl alcohol 

 combine to form the ester depends on the distance from the final state ; if one may 

 use a metaphorical expression, this final or equilibrium state is foreseen from the 

 very beginning. 



