40 LECTURE III. 



smaller without, however, undergoing any other perceptible 

 change (Fig. 9, 2). This diminution in size can only mean 

 that water has been withdrawn from the cells, the withdrawal 

 being accompanied by an elastic contraction of the cell-wall. 

 The nitre solution has withdrawn water until its attraction for 

 water has come to be equal to that of the cell-sap, and this 

 state of equilibrium having been reached, the withdrawal has 

 ceased. On replacing the nitre solution by distilled water, 

 the cells will regain their original size. From these very 

 simple experiments we learn that the cell is capable of 

 absorbing water in such quantity as to cause considerable 

 stretching of the cell-wall and of the primordial utricle, that 

 is, to set up a considerable hydrostatic pressure in the cell. 

 This state of tension between the hydrostatic pressure on the 

 one hand and the elasticity of the cell-wall on the other is 

 designated as ttirgidity. 



The diffusion of liquids through membranes is termed 

 osmosis, and we may now enquire into the conditions of its 

 accomplishment. They are briefly these. When two different 

 liquids are separated by a membrane which they are both 

 capable of wetting, currents are set up between the two liquids 

 which traverse the membrane. Thus when an ordinary osmo- 

 meter (the membrane of which consists of a piece of ox- 

 bladder with the muscular coats removed) containing a liquid, 

 is placed in a vessel containing another liquid, currents pass 

 through the membrane into and out of the osmometer, the 

 former being termed the endosmotic the latter the exosmotic 

 current. The quantity of the liquids conveyed in each of 

 these directions may be the same, or they may differ very 

 considerably, for it has been ascertained that that liquid 

 traverses the membrane in greatest quantity which wets it 

 most readily, and further, that the chemical nature of the 

 liquids is of great importance in the process. Thus Graham 

 observed that when an osmometer, with a bladder-membrane, 

 containing alcohol was introduced into a vessel containing 

 distilled water, the level of the liquid in the osmometer-tube 

 rose rapidly in consequence of the endosmose of water, where- 

 as, when a film of collodion was substituted for the bladder 



