258 THE PROPERTIES OF PROTOPLASM 



The pressure which is required to exactly compensate the greater 

 frequency of collisions on the side of the membrane which is bathed 

 by pure water is termed the Osmotic Pressure of the solution. Its 

 measurement may be rendered automatic by enclosing the solution in 

 a vessel to the orifice of which a Manometer is attached, so that the 

 entry of a very minute amount of water, into the vessel, insufficient 

 to appreciably dilute the solution, causes a very considerable rise in 

 the mercury column of the manometer, and a proportionately large 

 increase of pressure. With practically negligible dilution of the 

 solution, therefore, the necessary pressure is attained and cannot be 

 exceeded, because the os notic pressure having once been attained, the 

 rates of entry and exit of water into and oit of the vessel become 

 equal and no further changes of pressure or composition can occur. 



Such a vessel, provided with a semipermeable membrane and a 

 manometer, is termed an Osmometer. Since the pressures which are 

 generated are usually very great, the walls of the vessel must be made 

 of strong material, and the membrane, especially, must be constructed 

 so as lot to break under the strain. These desiderata are attained by 

 employing a vessel composed of earthenware, in the minute pores of 

 which membranes are formed by filling the vessel with a solution of 

 potassium ferrocyanide and immersing it in a solution of copper 

 sulphate. The two reagents diffuse outward and inward, or may be 

 induced to do so by electrolysis, until they meet at some point within 

 the pores across which membranes of minute dia leter are formed. 

 Such membranes withstand relatively enormous pressures, while a 

 large continuous membrane would rupture under the strain of much 

 smaller pressures. 



Another way of equalizing the rates of transposition of water across 

 the membrane would be to raise the Temperature of the solution 

 above that of the water upon the other side of the membrane. Increase 

 in temperature results in proportionate increase of the mobility of 

 the molecules, so that the collisions with the membrane would be 

 proportionately more numerous per unit of time on the heated than 

 on the unheated side. This procedure is not practicable with the 

 membranes which we have hitherto been considering, because they 

 would conduct heat from the one chamber to the other and the tem- 

 peratures of the two compartments on either side of the membrane 

 would soon be equalized. We may very easily employ this method, how- 

 ever, if in the place of a thin solid membrane we employ a layer of air. 

 Then, provided the dissolved substance is not volatile, i. e., soluble in 

 air, we have in effect a semipermeable membrane which is a poor con- 

 ductor of heat and which may be obtained of any thickness which we may 

 desire. If two chambers or vessels, the one containing water, the other 

 containing a solution, be both placed in a confined space or large vessel 

 filled with air, water will, if the two liquids are at the same temperature, 

 slowly distil over from the compart nent co itaining pure water into 

 the compartment containing the solution, which thus becomes pro- 



