358 HOW CEOPS GEOW. 



acid or alkali is carried beyond a point peculiar to each 

 substance, contraction of the colloid takes place. The 

 colloids just named acquire the power of combiniag with 

 an increased proportion of water and of forming higher 

 gelatinous hydrates in consequence of contact with dilute 

 acid or alkaline reagents. Even parchment-paper is more 

 elongated in an alkaline solution than in pure water. 

 When thus hydrated and dilated, the colloids present an 

 extreme osmotic sensibility." 



An illustration of membrane-diffusion which is highly 

 instructive and easy to iwoduce, is the following : 



A cavity is scooped out in a carrot, as in fig. 68, so that 

 I the sides remain ^ inch or so thick, and a 

 quantity of dry, crushed sugar is introduced ; 

 after some time, the previously dry sugar wUl 

 be converted into a syrup by withdrawing 

 water from the flesh of the carrot. At the 

 same time the latter will visibly shrink from 

 the loss of a portion of its liquid contents. In 

 this case the small portions of juice moistening 

 the cavity form a strong solution with the 

 sugar in contact with them, into which water diffuses from 

 the adjoining cells. Doubtless, also, sugar penetrates the 

 parenchyma of the carrot. 



In the same manner, sugar, when sprinkled over thin- 

 skinned fruits, shortly forms a syrup with the water which 

 it thus withdraws from them, and salt packed with fresh 

 meat runs to brine by the exosmose of the juices of the 

 flesh. In these cases the fruit and the meat shrink as a 

 result of the loss of water. 



Graham observed gum tragacanth, which is insoluble in 

 water, to cause a rapid passage of water through a mem- 

 brane in the same manner from its power of imbibition, 

 although here there could be no exosmose or outward 

 movement. 



The application of these facts and principles to explain- 



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Fig. 68. 



