162 NON-METALS AND THEIR COMBINATIONS. 



of determining molecular weights has been devised, but for a description 

 of the apparatus and details of working, reference must be made to special 

 books. 



The substances which show abnormalities in the depression of the freezing- 

 point are also those which give abnormal elevations in the boiling-points of 

 their solutions. The abnormal behavior is also accounted for by the same ex- 

 planation, namely, a decomposition of some of the molecules of the dissolved 

 substance. The deviations from normal behavior are particularly observed in 

 aqueous solutions. 



Osmotic pressure. Soluble substances in contact with a liquid dissolve 

 and diffuse throughout the liquid until the concentration is uniform in every 

 part of the solution (see Diffusion, p. 40). In the liquid the substance behaves 

 somewhat like a gas, in that its molecules tend to spread out and fill the whole 

 space occupied by the liquid. The cohesion between the molecules of the sub- 

 stance is overcome and there is freedom of motion, somewhat as in a gas. 

 Just as a gas exerts a pressure on any partition or membrane that resists the 

 motion of its molecules, so likewise do the molecules of a substance in solution 

 exert pressure upon a membrane that prevents their diffusion into a less con- 

 centrated region of the solution, or into the pure solvent. This pressure is 

 called osmotic pressure. In the article on Dialysis it is shown that a substance 

 in aqueous solution on one side of certain kinds of membranes, as bladder or 

 parchment, will diffuse into pure water on the other side. Osmotic pressure is 

 exhibited here, but such membranes are not suitable for its study, because the 

 motion (diffusion) of the molecules of the dissolved substance is not stopped, 

 but only hindered. It is possible, however, to prepare membranes which are 

 permeable to the solvent, but impermeable to the dissolved substance. These 

 are known as semi-permeable membranes, and by their means the phenomena 

 of osmotic pressure can be studied qualitatively and quantitatively. W. 

 Pfeffer, a botanist, was the first to successfully construct (1877) an artificial 

 semi-permeable membrane by causing a precipitate of copper ferrocyanide to 

 be formed within the walls of a porous unglazed porcelain cup. 1 Such cups 

 are known as osmotic cells. When a solution of any substance, say, cane-sugar, 

 is placed in the cell and the latter is placed in water, it is observed that water 

 passes through the cell into the solution, but no sugar passes out into the 

 water. If the flow of water is unobstructed, it will continue until the solution 

 is so much diluted that it is practically the same as water. If the accumulation 

 of water in the cell is obstructed by using a closed cell filled with the solution 

 and fitted with a manometer, pressure is seen to develop in the cell, due to the 

 tendency of the water to pass into it, and corresponding to the amount that 

 would have passed into it had the water not been obstructed. It requires some 

 hours for this pressure to reach a maximum, and the amount in atmospheres 

 can be read on the manometer. At the maximum the pressure on the water 

 within the cell causes it to tend to flow out as fast as the water outside tends 

 to flow in, thus producing a system in equilibrium. The pressure read on the 

 manometer is equal to the osmotic pressure of the molecules of the dissolved 

 substance against the membrane of the cell. 



1 It was rather difficult to prepare such membranes until a method was devised by Prof. 

 H. N. Morse, by which practically flawless osmotic cups can be readily made. See Amer- 

 Chem. Jour., July, 1905, and later. 





