CH. XXIV.] OSMOTIC PHENOMENA 323 



through the solution by the movement of the ions. Substances which exhibit the 

 property of dissociation are known as electrolytes. 



The liquids of the body contain electrolytes in solution, and it is owing to this 

 fact that they are able to conduct electrical currents. 



This conception of electrolytes which we owe to Arrhenius is extremely impor- 

 tant in view of the question of osmotic pressure, because the act of dissociation 

 increases the number of particles moving in the solution, and so increases the 

 osmotic pressure, for in this relation an ion plays the same part as a molecule. 



Another physiological aspect of the subject is seen in a study of the actions of 

 mineral salts in solution on living organisms and parts of organisms. Many years 

 ago Ringer showed that contractile tissues (heart, cilia, etc.) continue to manifest 

 their activity in certain saline solutions. 



Loeb and his fellow-workers have confirmed these statements, but interpret them 

 now as ionic action. Contractile tissues will not contract in pure solutions of non- 

 electrolytes (such as sugar, urea, albumin). But different contractile tissues differ 

 in the nature of the ions which are most favourable stimuli. Thus cardiac muscle, 

 cilia, amoeboid movement, karyokinesis, cell division, are all alike in requiring a 

 proper adjustment of ions in their surroundings if they are to continue to act, but 

 the proportions must be different in individual cases. 



Loeb at one time considered that the process of fertilisation was mainly ionic 

 action, but since then he has modified his views ; the action of ions is only one of 

 many factors. Howell's work, however, on the action of ions in the causation 

 of the heart-beat (see p. 261) may be taken as one of the best-proved instances 

 of the importance of this branch of study. 



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 sub- 

 stance per litre. Thus a gramme-molecular solution of sodium chloride is one which 

 contains 58'46 grammes of sodium chloride CNa = 23'00: 01 = 35*46) in a litre. A 

 gramme-molecular solution of grape sugar (C 6 H ]2 O 6 ) 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 was 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 '46 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 upon 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 homo- 

 geneous 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 large molecules (starch, pro- 

 tein, etc.) that they will not pass through such membranes are called colloids. 

 Diffusion of substances in solution which have to deal with an intervening membrane 



