230 PRINCIPLES OF GENERAL PHYSIOLOGY 



method of measuring these changes by the principle of interference of light waves. 

 A method has long been in use for many purposes, in which the refractive index of 

 a liquid is determined directly, by the amount of deviation through a prism ; but 

 the method by which changes in the refractive index are caused to produce 

 interference bands is far more delicate. Suppose that we have a train of light 

 waves, of a particular wave length, and that part of this passes through a column 

 of water, on the one hand, and another part through a solution of a substance 

 which slows the rate of transmission of light through it. The wave length will not 

 be the same in the two beams, so that, if they are combined together, the direction 

 of vibration, if coincident at one point, will be opposite at a certain number of 

 waves distant, where there is half a wave length difference between them. When 

 there is again a whole wave length difference, the directions are again coincident. 

 The result is a series of alternate dark and light bands. This brief description is 

 only intended to illustrate the principle on which the method is based. Details 

 of the construction of the instrument will be found in Lowe's papers (1910 and 

 1912). It will be clear that the changes in concentration to be measured must 

 affect one constituent of the solution only, unless those of other constituents are 

 related to this in a known way. The method can also be used, as originally by 

 Rayleigh, for the analysis of mixtures of gases, if the tension of one only varies 

 independently. The instrument, as made by Zeiss, determines the concentration 

 of solutions up to 8 per cent, sodium chloride with an error of 0'003 per cent, of 

 the solute, or, with a longer chamber, solutions between and 1 per cent, with an 

 error of O0004 per cent, in the salt. 



Hydration of Solute. As just mentioned, there is, at all events in a large 

 number of cases, combination of some kind or association between the molecules of 

 the solvent and those of the solute. Leaving out for the present the hydration of 

 ions, it must be admitted that the evidence for such hydration is mainly indirect, 

 and, in fact, Nernst (1911, pp. 271 and 537) appears to regard the hypothesis as 

 by no means proven. 



The meaning of the name "hydration" must be distinguished from that of hydrolytic 

 dissociation. The former refers to the combination of the molecules or ions of the solute with 

 the molecules of water as such. The latter, as already explained, is a decomposition of a salt 

 into free acid and base by interaction with the hydrogen and hydroxyl ions of electrolytically 

 dissociated water. 



The solubility of gases in water is diminished, not only by electrolytes, but 

 also by some non-electrolytes, and the most satisfactory way of accounting for 

 the fact is that the solute has in some way taken up a number of the molecules 

 of the water, leaving fewer to dissolve the gas. One molecule of saccharose, for 

 example, takes up six molecules of water (Philip, 1907). Carl Miiller (1912, 

 p. 502) finds that the diminution of solubility of a gas by a given solute is 

 independent of the chemical nature of the gas. This can only be explained by 

 an influence of the solute on the solvent, and most readily by the formation of 

 " hydrates." This phenomenon of hydration may possibly play a part in the 

 effect of neutral salts on the activity of an acid in the inversion of cane-sugar. 

 It is clear that, if the neutral salt takes up a number of the molecules of the 

 disposable water, the acid present will be in higher concentration in the remainder. 

 It seems, however, doubtful whether this effect is capable of accounting for the 

 whole of the apparent increase in the concentration of H' ions (see also page 195 

 above). 



Considerable evidence has been brought by Jones (1907) and by Jones and 

 Anderson (1909) in favour of the hydration of salts in solution. If this takes 

 place, it is generally supposed to be an equilibrium of such a kind that the 

 more water present, relatively to the solute, the more molecules of it are associated 

 with each molecule of the latter. Now, the absorption of light by solutions of 

 substances is, by Beer's law (see Chapter XIX.), proportional to the number 

 of molecules through which the rays pass. Further, if water molecules are taken 

 up, it is to be expected that the vibration period and other properties of the 

 molecules of the solute will be found to be different according to the dilution. 

 Fig. 67 shows four series of photographs of absorption spectra of solutions of 



