M. Dutrochet's Discoveries in Vegetable Physiology. 107 



brane, there are formed across this membrane two currents of 

 opposite directions and of unequal force. It follows that the 

 fluid accumulates in greater quantity at the side towards which 

 the strongest current is directed. These two currents exist 

 in the hollow organs which form organic textures, and it 

 is there that I have described them under the names of en- 

 dosmose and exosmose. My experiments have shown me that 

 this phenomenon is not exclusively produced by organic mem- 

 branes. Porous inorganic plates very thin produce them also. 

 The extreme thinness of the permeable membrane is a neces- 

 sary condition for producing the phenomenon, which does not 

 show itself for example when the permeable membrane is four 

 millimetres in thickness, but which takes place when the mem- 

 brane is only one millimetre in thickness. In these two cases, 

 however, the porous plates will have an equal capillary action, 

 that is to say, they will be susceptible of transmitting by fil- 

 tration an equal quantity of water in a given time. The 

 proximity of the two heterogeneous fluids appears thus to be a 

 necessary condition for producing the phenomenon, which does 

 not depend upon capillarity alone, as a celebrated mathemati- 

 cian has maintained, and of whose theory I shall now give a 

 summary account. * When two fluids of different densities, 

 and whose heights are in the inverse ratio of their densities, 

 are separated by a membrane whose capillary pores are per- 

 meable to these fluids, the pressures exerted upon the orifices 

 of these pores are equal on every side ; but the capillary force 

 being unequal at the two ends of the pore, it follows that the 

 fluid exposed to the strongest capillary action fills the whole 

 pore. This filament of fluid is therefore acted upon by two 

 opposing forces ; 1st, the attractions of the fluid to which it 

 belongs ; 2d, the attraction of the other fluid situated at the 

 opposite side. But this last attraction being greater than the 

 first, it follows that the filament of fluid contained in the ca- 

 pillary pore will flow without stopping in the direction in 

 which it is acted upon by the stronger attraction, and will in- 

 crease also continually the fluid mass towards which it is drawn. 



* On the effects which may be produced by the capillarity and affinity 

 of heterogeneous substances, by M. Poisson, in the Annates de Chimie, <%c. 

 torn. xxxv. p. 98. 



