i;S PRINCIPLES OF GENERAL PHYSIOLOGY 



This slow movement of ions allows another piece of evidence to be brought in favour of the 

 actual existence of ions in solutions of electrolytes apart from any passage of current through 

 them. Take a solution of copper sulphate and place in it two elect rodes at '2"1 cm apart. 

 Let the anode consist of copper and the cathode of platinum. As soon as the current is 

 established, copper is deposited on the platinum plate and dissolved from the anode by the 

 >SO 4 " ion. Now, if. the electrical current itself split up the CuS0 4 molecules and the two 

 oppositely charged parts were attracted to the two opposite poles, according to the old view, 

 it follows that the >S0 4 " ion belonging to a particular copper ion at the cathode has to travel 

 in our case 2^2 cm. in less than one second. Suppose that the potential difference were 

 2 - 2 volts and that we ascribe to the SO 4 " ion as great a velocity as that of the OH' inn 

 (G'0018 cm. per second) (it is really much less), twenty minutes will be required for it to 

 travel the distance of 2'2 cm. between the electrodes. 



Ostwald (1888, p. 272) directs attention to another similar experiment. It is well known 

 that, if amalgamated zinc be immersed in dilute sulphuric acid, it is not attacked. But if 

 a piece of platinum be also immersed in the same solution, even at a considerable distance, 

 as soon as the two metals are connected by a wire, hydrogen appears on the platinum and 

 zinc goes into solution. The hydrogen cannot arise from the same sulphuric acid molecule 

 whose S0 4 attacks the zinc, since it cannot travel the distance in the time. It must come 

 from the immediate neighbourhood and have been already present as dissociated ionic 

 hydrogen. 



Another fact which is readily explained by the different rate of migration of 

 ions already present and for which no other explanation is at hand, is that, when 

 a solution of an electrolyte is in contact with water, a potential difference is nearly 

 always found to exist at the boundary surface. This is due to the unequal rate of 

 diffusion of the two ions, so that either the anion or the cation is in advance of 

 the other, forming a Helmholtz double layer. Of course, they cannot separate far 

 from one another, on account of electrostatic attraction. We shall meet with this 

 phenomenon again in connection with the sources of electrical changes in living 

 ' tissues. 



HYDRATION OF IONS 



When we look at the numbers in the table of ionic conductivities on page 177 

 above, we are struck by the fact that lithium, with an atomic weight of 7, moves 

 at a much slower rate than potassium, with an atomic weight of 39. The 

 explanation is probably that the lithium ion carries with it a larger number of 

 water molecules than the potassium ion, so that greater friction is experienced. 



The chief work on this question has been done by Bousfield (1905, 1906, 1912), to whose 

 papers the reader is referred. An interesting fact, which is worth quoting, comes out from 

 the results of the last paper (1912, p. lb'8). The number of molecules of water combined with 

 both ions at infinite dilution is for 



KC1 NaCl LiCl 



9 13 21 



There are reasons for supposing that the number combined with the Cl' ion is f>, since 

 its transport number is just a little greater than that of potassium, so that its share must be a 

 little more than half of the total 9 of KC1. If this is so, we have for the number of molecules 

 of water associated with the ions of 



K Na Li 



4 8 16 



As the author says, ' ' an attractive looking series. " 



FURTHER EVIDENCE AND SOME DIFFICULTIES 



The chief evidence for the truth of the electrolytic dissociation theory is, 

 undoubtedly, the fact that it is capable of giving correct quantitative explanation 

 of so many phenomena, and even of predicting the numerical values of the factors 

 in these phenomena. It is not surprising that deductions from it have not always 

 been verified, since modifications and additions are always necessary in theories 

 of such far-reaching application. 



Objections have been brought against it, but no rival theory has been shown 

 able to afford the accurate quantitative results that it does in so simple and direct 

 a manner. At the present time it may safely be said to be indispensable. There 

 are many phenomena which, without it, could not even be described except with 

 difficulty, much less treated quantitatively. Of these we shall presently meet with 

 some striking examples. 



