232 PRINCIPLES OF GENERAL PHYSIOLOGY 



copper chloride in water. In A, the concentration increases from 0'562 molar 

 to 4'5 molar, from above downwards, and the depth of the solution is varied 

 inversely with the concentration, so that the same total amount of solute lies 

 in the path of the light. It is seen that the more dilute the solution, the It >> 

 ultra-violet is absorbed. In B, we have a similar series with more dilute solutions. 

 In C and D, the concentrations are chosen so as to compensate for increa>rd 

 dissociation on dilution, so that the number of undissociated molecules in the 

 path of the light should be constant. A similar effect is seen. There are two 

 ways of accounting for the increase in absorption with concentration, when the 

 number of molecules is kept constant. Aggregates may be formed and the 

 absorbing power increased thereby ; or solvates may be formed, in proportion 

 to dilution, and the absorbing power decreased with increase in number of 

 molecules of water taken up. To decide between the two views, we can test 

 the effect of rise of temperature, which breaks up aggregates. The effect is 

 the same as that of increasing concentration ; hence it is to be concluded that 

 the action of increased concentration on the absorption of light is not due to 

 aggregation of solute, which would have the opposite effect. The concentration 

 of water, in the experiments in question, was also varied by the addition of 

 calcium chloride or alcohol, and the salts of several different metals were 

 investigated, with results similar to those mentioned. 



An important case for the physiologist is the state of amino-acids in water. 

 Winkelblech (1901, p. 590) points out that taurine (amino-ethyl sulphonic 

 acid) forms no salt with hydrochloric acid, and that it is usually supposed to form 

 a ring compound, internal anhydride, or internal salt, in water. Might it not 

 also be that the sulphonic acid group makes it too strong an acid, even when 

 partially counteracted by the NH., ? In the case of the ordinary carboxylic amino- 

 acids, even supposing that such internal salts are formed by combination of the 

 NH., and COOH groups with one another, as salts of very weak acids and bases, 

 they will be greatly dissociated hydrolytically in water, according to the laws 

 given on page 198 above. In fact, as Winkelblech shows (1901, p. 592), glycine, 

 according to the equation of Arrhenius, must be hydrolytically dissociated to 

 the extent of 99*967 per cent. ; this proportion is present as hydrated 

 glycine, the smaller remainder as internal salt together with a few ions. It 

 is therefore present in solution practically entirely as 



/NH 3 OH /NHo 



CH< notasCH 2 < | 



"\COOH ' \COO 



The fact that taurine and the corresponding carboxylic acid, alanine, have the 

 same very small electrical conductivity shows that electrolytic dissociation is 

 extremely low ; one would expect that the presence of the strongly acid sulphonic 

 acid group would give rise to considerably more H' ions than the carboxyl group. 

 This is one of the numerous cases that show that the chemical properties of a 

 particular group are not fixed, but depend on other constituents of the whole 

 molecule. 



The relation of lyophile colloids to the solvent has been treated of above (page 97), so that it 

 is unnecessary to do more than remind the reader of the facts, in connection with the 

 properties of water. 



DIELECTRIC CONSTANT AND ELECTROLYTIC DISSOCIATION 



In the previous chapter the relation of the dielectric constant to electrolytic 

 dissociation has been discussed and the fact pointed out that water has a higher 

 dielectric constant than any other solvent, with the exception of prussic acid and 

 hydrogen peroxide. Even where electrolytic dissociation is produced by other 

 solvents, the process appears to be a very complex one compared to the simple 

 splitting of the majority of salts in water. Association of solvent and solute 

 seems to occur to a large extent, as well as between the molecules of the solute 

 itself. This latter fact reminds us of the state of affairs in electrolytically 

 dissociated colloids in water, as described on page 160 above. 



