August 17, 1905] 



NA TURE 



383 



a gram of liquid water at the freezing point contains tlie 

 heat energy of 2155 calories. The fact that water has 

 the high vapour pressure of 46 mm. of mercury at the 

 freezing point is probably a result of this enormous store 

 of energy. As a liquid, therefore, it is natural to expect 

 that its molecules will exhibit effects proportionate to this 

 great store of energy. This expectation appears to be 

 realised %vhen we consider not only its properties as the 

 imiversal solvent, but its osmotic and diffusive energy in 

 solutions in which it is the solvent. 



To complete the comparison it is only necessary to 

 calculate the heat energy of gold at 0°. Taking its specific 

 heat as 0032, a gram of gold at 0° contains 87 calories. 

 A gram-molecule, therefore, contains in round numbers 

 1700 calories as compared with 3880 calories in a gram- 

 molecule of water. 



Taking into consideration not only this greater store 

 of energy, but also the much smaller cohesive force of water 

 as corjipared with the majority of solid solutes, there can 

 be no doubt that the active role in aqueous solutions of 

 this type must be assigned to the solvent, not to the solute 

 molecules. 



This leads to the important conclusion that the energy 

 of solution, of diffusion, and of osmosis is due, not to the 

 imaginary gaseous energy of the solute, but to the actual liquid 

 energy of the solvent molecules. When this conclusion is 

 reached a new physical explanation of these phenomena is 

 in our hands, and we are relieved from the strain to the 

 imagination involved in the application of the gas theory 

 to solutions of non-volatile solids. 



This transference of the active role to the solvent mole- 

 cules does not in any way affect the well-established con- 

 clusions based on the laws of thermodynamics as to the 

 energy relations in these phenomena, for it has always 

 been recognised that these conclusions have reference to 

 the average conditions prevailing in large collections of 

 relatively minute units. Wherever the gas analogy has 

 appeared to hold it has not necessarily involved more than 

 this, that the observed effects are in proportion to the 

 number of these minute units in a given volume. 



In applying the gas theory to the physical explanation 

 of osmotic pressure it has been the custom to regard this 

 pressure as directly due to the bombardment of the semi- 

 permeable membrance by the solute molecules. But this 

 conception completely ignores the fact that the pressure 

 developed is a hydrostatic, not a gaseous pressure, and 

 that the hydrostatic pressure results directly ^rom the 

 penetration of the solvent molecules from the other side 

 of the partition. 



It appears to me more natural to abandon the gas 

 analogy altogether, to regard the molecules as in the solid 

 and liquid condition proper to their temperature, and to 

 apportion to them their respective parts in the active 

 changes according to their obvious endowment of energy. 



Applying this view to the case of a solution and a solvent 

 separated by a semi-permeable membrane, it is seen that 

 ihe pressure rises on the solution side, because the pure 

 solvent molecules on the other side have some advantage 

 for the display of their energy over the similar molecules 

 in the solution. This effect in its most general form may 

 he attributed to the dilution of the solvent by the solute 

 molecules. In cases where the osmotic pressure appears 

 to obey Boyle's law the effect is exactly measured by the 

 number of solute molecules per unit volume. But the facts 

 of this position are in no way changed if the effect is 

 taken to be due to the activity of an equal number of 

 solvent molecules, for we then see that each solute molecule 

 by cancelling the activity of one solvent molecule on the 

 solution side permits a solvent molecule from the other 

 side to enter the solution. 



What the exact mechanism of this cancellation is there 

 is at present no evidence to show, and the caution origin- 

 ally given by Lord Kelvin with reference to the undue 

 forcing of the gas analogy must also be applied to the 

 suggestion now put forward. But as a means of making 

 the suggestion a little more clear I give here a simple 

 diagram on which a represents a single perforation in a 

 semi-permeable membrane, p, on both sides of which there 

 is only pure solvent. For the sake of clearness the mole- 

 cules are shown only as a single row. Normally there will 

 be no passage of solvent molecules from side to side, for 



N 1. 1868, VUL. 72] 



the average kinetic energy of the molecules on both sides 

 is equal. This state of equilibrium is indicated on the 

 diagram by marking with a cross the molecule which is 

 exactly halfway through the partition. 



At B a single solute molecule, s, has been introduced at 

 the right side. If this molecule exactly cancels the energy 

 of one solute molecule at its own end of the row, the 

 equilibrium point will move one molecule to the right, the 

 solvent molecules will move in the same direction, and 

 one of their number will enter on the solution side. So 

 long as the row includes one, and only one, solute molecule, 

 the equilibrium will remain unchanged and no more solute 

 molecules will pass in. If another solute molecule arrives 



ooooeoooo 



on the scene, the equilibrium will again be disturbed in 

 the same way as before, and another solvent molecule will 

 pass into the solution. 



This mechanism accomplishes to some extent the work 

 of a " Ma.xwell Demon," in so far at least as it takes 

 advantage of the movement of individual molecules to raise 

 one part of a system at a uniform temperature to a higher 

 level of energy. 



A Mechanical View of Dissociation in Dilute Solutions. 

 The view that the phenomena of solution depend on the 

 relative kinetic energy of the solvent and solute molecules 

 appears to apply with special force to the phenomena of 

 dissociation in dilute solutions. Under the gas theory there 

 does not appear to be any reason why the solute molecules 

 should dissociate into their ions. So obvious is this absence 

 of any physical motive that Prof. .Armstrong has 

 happily referred to the dissociation as " the suicide of 

 the molecules." Others have proposed to ascribe the phe- 

 nomenon to what might be called " the fickleness of the 

 ions," thus supposing that the ions have an inherent love 

 of changing partners. These may be picturesque ways of 

 labelling certain views of the situation, but the views 

 themselves do not appear to supply any clue to the physical 

 nature of the phenomena. With the acceptance of the 

 view that the phenomena of solution are largely due to the 

 kinetic energy of the solvent molecules, the phenomena of 

 dissociation also appear to take their place as a natural 

 result of this activity. For consider the situation of an 

 isolated molecule of cyanide of gold and potassium closely 

 surrounded by and at the mercy of some millions of water 

 molecules all in a state of intense activity. The rude 

 mechanical jostling to which the complex molecule is sub- 

 jected will naturally tend to break it up into simpler 

 portions which are mechanically more stable. The me- 

 chanical analogy of a ball mill in which the balls are self- 

 driven at an enormous velocity is probably rather crude, 

 but it may at least help us to picture what, on the view 

 now advanced, must be essentially a mechanical operation. 



In importing this mechanical view of the breaking down 

 of complex into simpler molecules we are not without 

 soine solid basis of facts to go upon. My own observ- 

 ations have shown that even in the solid state the crystalline 

 molecule can be broken down by purely mechanical means 



