,^8 



NA TURE 



[August 19, 1909 



importance of the effects of osmotic pressure when it 

 comes inio play, but it puts the phenomena in their true 

 light as consequences of the law of vapour-pressure. 



Regarded as a verification of the laws of vapour-pressure, 

 direct measurements of the osmotic pressure are of the 

 highest value, but there are comparatively few cases known 

 at present in which such direct measurements are possible. 

 In other cases the osmotic pressure, if it exists, can 

 always be calculated from a l^nowledge of the vapour- 

 pressure. For the elucidation of osmotic phenomena and 

 many other problems in the theory of solutions we are 

 compelled to make a systematic study of the relations of 

 vapour-pressure. Much has been done in this direction in 

 the past, but, owing to the difficulty of the measurements, 

 much remains yet to do. I may, therefore, be pardoned 

 if I allude brieflv to some of the methods which I have 

 employed for this purpose, and some of the conclusions 

 at which I have so far arrived. 



It is often a difificult matter, when the difference of 

 vapour-pressure between a solution and the solvent is 

 small, to measure the pressure difference directly to a 

 suflficient degree of accuracy. A method very commonly 

 emplo^■cd, which has been brought to a high degree of 

 accuracy by Lord Berkeley and his assistants, depends on 

 the observation of the losses of weight of two vessels, 

 containing solution and solvent respectively, when the 

 same volume of air is aspirated slowly through them in 

 succession. To .secure accurate results, the air must pass 

 very slowly. One complete observation t.ikes about a 

 week to perform successfullv. and involves manv difficult 

 manipulations. I have endeavoured to avoid this difficulty 

 by measuring the temperalure difference in place of the 

 pressure difference, since the temperature difference re- 

 mains nearly constant, while the pressure difference tends 

 to diminish in geometrical progression with fall of 

 temperature. The method adopted for this purpose is that 

 indicated in the diagram of the vapour-temperature 

 balance. The temperatures of solution and solvent, con- 

 tained in seoarate vessels communicating through a tap, 

 are adjusted until, on opening communication between 

 them, there is no flow of vapour from one to the other, 

 as indicated by a change in the reading of a pair of 

 thermojunctions immersed in the solvent respectivelv. The 

 corresoonding diffrrence of temperature is observed, and 

 since the vapour-pressures of the solvent are known, it is 

 easy to calculate the required ratio or difference of the 

 vapour-pressures of solvent and solution at the same 

 temperature. When the vapour-pressures are very small 

 it may be difificult to observe the change of temperature 

 on opening the tap unless the apparatus is verv carefullv 

 exhausted. \ more delicate method in this case is to 

 observe the direction and magnitude of the current of 

 vapour from solution to solvent, or vice versA, bv means 

 of the " vapour-current indicator," illustrated in "the com- 

 panion diagram. This consists of g delicatelv suspended 

 vane, the deflections of which are read by a mirror, and 

 will readily indicate a difference of pressure less than the 

 thousandth part of a millionth of an atmosphere. 



The vapour-current indicator is so constructed that its 

 deflections are very accurately proportional to the nressure 

 difference, much more so, in fact, than anv form of 

 electric galvanometer. It can also be employed for direct 

 measurements of small differences of vapour-pressure. 

 The chief difficulty in this case is to ensure the absence 

 of air or other disturbing factors. A method of avoiding 

 this difficulty is to work at atmospheric pressure, and to 

 measure the pressure difference between two vertical 

 columns of air saturated with the vapours of the solvent 

 and solution respectively.' The temperature difference may 

 be adjusted (o balance, and is preferablv measured by 

 means of a pair of differential platinum thermometers, 

 which permits a higher order of accuracy to be attained 

 than the thermoelectric method. 



Vapour-pressure in relalion to Molecular Constitution. 



The well-known law of Raoult, according to which the 

 relative lowering of vapour-pressure of a solution is equal 

 to the ratio of the number of molecules n of the solute to 

 the number of molecules of solvent N in the solution, has 



I firsf showed th's expe'imenl ten year-; ago, in illustration of the deli- 

 cacy of the appar.-ttus, at a Friday Evening Lecture at the Royal Institution. 



NO. 2077, VOL. Si] 



thrown a great deal of light on the molecular state of 

 the dissolved substance in dilute solutions, but fails notably 

 in many cases when applied to strong solutions. In the 

 case of homogeneous mi.xtures of two indifferent volatile 

 substances, such as benzol (CjHj) and ethylene chloride 

 (C„H|CI.), which mix in all proportions without mutual 

 action, a slightly different but equally simple law is known 

 10 hold verv accurately throughout the whole range of 

 concentration from o per cent, to loo per cent. The 

 vapour-pressure of each ingredient is simply proportional 

 to its molecular concentration. In other words, the ratio ' 

 of the partial vapour-pressure p' of either constituent at 

 any concentration to its vapour-pressure />„' in the pure 

 state at the same temperature is equal to the ratio of 

 the number of its molecules n' in the solution to the whole 

 number of molecules n' + n" of both substances in the 

 solution. Such is evidently the form of the simple mixtur» 

 law. For substances which form compounds in the solu- 

 tion, or the molecules of which are associated or dis- 

 sociated, this simple law is widely departed from. In a 

 recent paper, " On Vapour-pressure and Osmotic Pressure 

 of Strong Solutions " (Proc. R.S.A., vol. Ixxx., p. 466,' 

 igoS), I have endeavoured to extend this simple relation' 

 to more complicated cases by making the obvious assump- 

 tion th.at, if compound molecules are formed, they should 

 be counted as single molecules of a separate substance in 

 considering their effect on the vapour-pressure. With 

 this proviso the vapour-pressures of strong solutions are 

 well represented by a natural extension of the siinple mix- 

 ture law, and it becomes possible to investigate the nature 

 of the compounds formed in any case. To take a simple 

 Instance, suppose that each of the 11 molecules of the dis- 

 solved substance combines with a molecules of the solvent, 

 the total number of molecules of the solvent being N. 

 The ratio of the vapour-pi=essure fi" of the solvent in the 

 solution to the vapour-pressure /)' of the pure solvent at 

 the same temperature will then be the same as the ratio 

 of the number N—nii of molfculcs of free solvent in the 

 solution, to the whole number of inolecules N — on-^« in 

 the solution, each compound molecule being counted as a 

 single molecule. 



With the simple formula 



p';p" = (S -an + n)l{}i-an). 



the values of the vapour-pressure are very easily calculated 

 from the molecular concentration n for simple integral 

 values of the hydration factor a. The osmotic pressures 

 are also readilv deduced from the ratio of the vapour- 

 pressures ip' : p") bv the formula 



PU = RT log (/>'//)"). 



The value a = ~, fits the osmotic pressures for cane-sugar 

 very well, as shown in the column headed C in Table I. 

 The value a = 2 fits Lord Berkeley's observations on 

 dextrose equally well up to pressures of 130 atmospheres. 

 The same value = 5 for cane-sugar also fits the observa- 

 tions on the depression of the freezing poinfj and the rise 

 of the boiling point, as it necessarily must, since these 

 phenomena also depend on the vapour-pressure. The 

 freezing-point method is the easiest for getting the ratio 

 of the vapour pressures to compare with the formula. At 

 the freezing point of an .^queous solution the vapour- 

 pressure of the solution must be the same as that of ice, 

 provided that ice separates on freezing in the pure state. 

 The ratio of the vapour-pressure of ice to that of water 

 at anv temperature below 0° C. is easilv calculated. All 

 the best recorded results, except those of a few associating 

 substances, give simple positive integral values of a. 

 Even in the case of associating substances, like formic 

 acid and acetone, the curves are of the same type, but 

 the value of a is negative. Dissociating substances, like 

 strong electrolytes, present greater difficulties, on account 

 of the ionisation factor; but, allowing for the uncertainty 

 of the ionisation data, they seem to follow satisfactorily 

 the same law of vapour-pressure. 



It appears from the form of tfe proposed i,',w that the 

 hydration factor a makes very little difference tj the 

 vapour-pressure in weak solutions, which follow Raoult 's 

 law as a limiting case, but it makes a very great differ- 

 ence in strong solutions, when nearly all the free water 

 is used up, and the denominator X — ai! is small. Thus 



