238 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



tion must be left to the future. So also must the equally important idea 

 that the basis of a nervous impulse is electrolytic action, though it receives 

 support from Macdonald's recent investigations. 



Gramme-molecular Solutions. From the point of view of osmotic pressure 

 a convenient unit is the gramme -molecule. A gramme -molecule of any 

 substance is the quantity in grammes of that substance equal to its molecular 

 weight. A gramme-molecular solution is one which contains a gramme- 

 molecule of the substance per litre. Thus a gramme-molecular solution of 

 sodium chloride is one which contains 58'5 grammes of sodium chloride 

 (Na = 23*05 ; Cl = 35*45) in a litre. A gramme-molecular solution of grape 

 sugar (C^HjoOJ is one which contains 180 grammes of grape sugar in a 

 litre. A gramme-molecule of hydrogen (H 2 ) is 2 grammes by weight of 

 hydrogen, and if this were compressed to the volume of a litre it would be 

 comparable to a gramme -molecular solution. It therefore follows that a 

 litre containing 2 grammes of hydrogen contains the same number of 

 molecules of hydrogen in it, as a litre of a solution containing 58*5 grammes 

 of sodium chloride, or one containing 180 grammes of grape sugar has in 

 it of salt or sugar molecules respectively. To put it another way, the heavier 

 the weight of a molecule of any substance the more of that substance must 

 be dissolved in the litre to obtain its gramme-molecular solution. Or still 

 another way : if solutions of various substances are made all of the same 

 strength per cent., the solutions of the materials of small molecular weight 

 will contain more molecules of those materials, than the solutions of the 

 materials which have heavy molecules. We shall see that the calculation of 

 osmotic pressure depends on these facts. 



Diffusion, Dialysis, Osmosis. If two gases are brought together within a 

 closed space, a homogeneous mixture of the two is soon obtained. This is 

 due to the movements of the gaseous molecules within the confining space 

 and the process is called diffusion. In a similar way diffusion will effect in 

 time a homogeneous mixture of two liquids or solutions. If water is carefully 

 poured on to the surface of a solution of salt, the salt or its ions will soon be 

 equally distributed throughout the whole. If a solution of albumin or any 

 other colloidal substance is used instead of salt in the experiment, diffusion 

 will be found to occur much more slowly. If, instead of pouring the water 

 on to the surface of a solution of salt or sugar, the two are separated by a 

 membrane made of such a material as parchment paper, a similar diffusion 

 will occur, though more slowly than in cases where the membrane is absent. 

 In time, the water on each side of the membrane will contain the same 

 quantity of sugar or salt. Substances which pass through such membranes 

 are called crystalloids. Substances which have such heavy molecules 

 (starch, protein, &c.) that they will not pass through such membranes are 

 called colloids. Diffusion of substances in solution in which we have to deal 

 with an intervening membrane is usually called dialysis. The process of 

 nitration (i.e. the passage of materials through the pores of a membrane 

 under the influence of mechanical pressure) may be excluded in such experi- 

 ments by placing the membrane (M) vertically as shown in the diagram 

 (fig. 83), and the two fluids A and B on each side of it. Diffusion through a 

 membrane is not limited to the molecules of water, but it may occur also in 



