CAUSES OF THE MOTION OF JUICES. 397 



"It is well known that fibrin, albumin, and animal 

 membrane swell much more in very dilute acids and 

 alkalies than in pure water. On the other hand, when 

 the proportion of 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 combining with an increased proportion of water 

 and of forming higher gelatinous hydrates in conse- 

 quence 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 produce, is the following : 

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

 that the sides remain inch or so thick, 

 and a quantity of dry, crushed sugar is 

 introduced ; after some time, the previ- 

 ously dry sugar will 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 por- 

 tion of its liquid contents. In this case 

 Fig. 68. foe 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 



