160 



PHYSIOLOGICAL BOTANY. 



scopicity of the vegetable membrane, we may here men- 

 tion a property of all membrane, which has probably a 

 considerable influence in the economy both of animal 

 and vegetable life. When a membrane is viewed under 

 the highest powers of the microscope, it appears to 

 possess a perfectly homogeneous texture, without pores 

 of any kind ; and yet water, milk, and other fluids, 

 placed under certain circumstances, are capable of pass- 

 ing through it with considerable facility. The con- 

 ditions required for producing this effect are these : 

 Any two fluids which exert a mutual affinity towards 

 each other, being placed on opposite sides of a mem- 

 brane, their immediate intermixture will commence, 

 each of them passing through the substance of the 

 membrane. If, for instance, a little treacle be enclosed 

 in a piece of bladder, and this immersed in water, a 

 portion of the treacle will soon be found to haveexudi <!. 

 whilst a still larger quantity of water will have pene- 

 trated into the bladder ; and this action will continue 

 until the fluids have acquired the same density. The 

 remarkable circumstance attending this phenomenon , i< 

 the fact of the lighter fluid having penetrated the mem- 

 brane with greater velocity 

 than the denser fluid. In 

 consequence of this, the 

 bladder becomes distended. 

 By a simple contrivance, 

 styled an endosmometer, 

 we may measure the degree 

 of force or velocity by 

 which the current of water 

 exceeds that of the current 

 of the denser fluid. In 

 fig. 148 A is a glass 

 funnel with the mouth 

 downwards, and covered 

 with a piece of bladder. 



The other end of this funnel is furnished with a tube 

 twice bent, the stems of which are vertical ; treacle 



