16 BIOLOGY. [Book i. 



feebly and slowly, as the following table indicates, whieli gives 

 the time of equal diffusion for some bodies taken in the two 



Ohlorohydric acid • 1 



GhisHwe of— sodium 2, 33 



Cane-sugar 7 



Sulphate of magnesia 7 



Albumine 49 



Caramel 98 



Herefrom' it is seen that ohlorohydric acid traverses the porous 

 membranes forty -nine times faster than albumine, and ninety-eight 

 times faster than caramel. It is no doubt owing to this feeble 

 diffusibility that the colloids are savourless when they are pure. 

 Besides, these colloids do not comprehend merely the complex 

 organic substances called al buminoid s ; certain bodies indubitably 

 mineral, such as silica, hydrated peroxide of iron, can assume 

 the colloidal condition. Both the one and the other, moreover, 

 enter into the composition of organised bodies. A particular 

 soluble form of the hydrated peroxide of iron, which normally is 

 an element of the blood, gives, when we dissolve it in water in 

 the proportion of 1 to 100, a red liquid, condensing into coagulum, 

 into a sort of rutilant clot, under the influence of traces of acids, 

 of alkalies, of alkaline carbonates, and of neutral salts. 



Certain colloids, such as gelatine, gum arabic, are soluble in 

 water ; certain others, such as gum tragacanth, are insoluble 

 therein. In any case they have as a general characteristic the 

 power of absorbing a great quantity of water, of augmenting 

 enormously in volume, and of then losing this water very 

 rapidly by evaporation. It seems as if in this case they are 

 merely subject to a sort of capillary imbibition. Yet it must be 

 admitted that they incorporate more intensely with themselves 

 a certain quantity of water as an integrant portion. Hence it 

 appears that there is for colloids a waitr of gelatirdsation as there 

 is for crystals a water of crystallisation. 



