Principles of General Physiology

The behaviour of ions as regards combination with water is similar to that of solutes in general. The fact has been referred to in previous pages in various connections, so that it is unnecessary to discuss the question further, except to call attention to an interesting paper by Kohlrausch (1902). This investigator found that the rates of migration of different ions approached nearer to the same value as the temperature was raised. Above the normal boiling point of water the effect is still more obvious, as appears from the following table of Noyes and Coolidge (1907, p. 47) :—

If drawn in curves, these results show that the mobilities would be identical at 360° C., that is practically at the critical temperature of water. At low temperatures, therefore, the sodium ion is the more bulky and for that reason the slower in movement, on account of the fact that it has more water molecules associated with it than the potassium ion has. But at high temperatures, owing to the loss of water, the two approximate to equal size and mobility.

It might perhaps be supposed that the considerations of the previous pages would invalidate conclusions with regard to osmotic pressure, since the concentration of the solvent is diminished by the amount of it which is taken up by the solute, so that the effective concentration of the solute would be increased. It is pointed out by Nernst (1911, p. 271) that it is not found experimentally that any anomalies result from this cause. The reason will be apparent from the following table given by Bousfield and Lowry (1910, p. 21) :—

It will be noted that the proportion of water of hyd ration to total water diminishes rapidly with the concentration and that it is only in the high concentrations that it would be detectable, owing to the very small amount of water taken up at the lower concentrations. Even in 0-2 molar concentration 97 per cent, of the water is free. Osmotic pressure, on the kinetic theory, being dependent on the energy of movement of the molecules of the solute, it is clear that a certain degree of polymerisation of the solvent, by which the total number of its molecules is decreased, will not have any obvious effect on the osmotic pressure of the solute.

The more carefully water is purified, the less is its power of conducting an electric current ; so that the conclusion must be made that it is, at the most, only very slightly dissociated into ions. H- and OH', therefore, can only exist beside one another in the merest traces. We have seen above the importance of this fact in the process of neutralising a base with an acid, and how, in consequence, the heat of neutralisation of a strong acid by a strong base is the same, whatever the acid or base used.

Now, since the conductivity of ordinary distilled water is readily shown to be due to impurities, it would seem that the view taken by Armstrong, that if water were sufficiently pure it would be a non-conductor, is a justifiable one. It is obvious that the argument cannot be disproved by direct measurements of the conductivity of purified water. At the same time, the results of Kohlrausch and Heydweiller (1894) distinctly point to a limit, beyond which further purification has no effect. These experiments give a concentration of l'05x!0~7 gram-ions per litre at 25°, or 0-78 x 10~7 at 18°. This gives a value for the product of ionic concentrations (H«) (OH') of M x lO"14 at 25°.

The substantial correctness of the view, moreover, is shown by the fact that other independent methods give values almost identical with this. 1. The addition of large quantities of a strong alkali to water will render infinitesimal the concentration of any free hydrogen ions arising from the dissociation of an acid present as impurity ; so that, if the presence of any such ions can be detected, they must arise from the water itself. This can be done by taking the electromotive force of a battery of acid and alkali by the method of Nernst described above (page 191). The value of the concentration in hydrogen ions found in this way was 0'8 x 10~" at 19°.

2. We have seen how satisfactorily the hydrolytic dissociation of certain salts in water is explained by the existence of H* and OH' ions in water. This is, in itself, evidence for the truth of the hypothesis, but the numerical "value of the dissociation of water can be calculated from the degree of hydrolysis of a solute and 0-68 x 10~" has been found in this way. 3. In the chapter on " Catalysis " we shall see how acids cause an increased rate of hydrolysis of esters in water and how alkalies cause an increased rate of saponification. So that, if the rates of these reactions in pure water be determined, we have another means of arriving at the concentration of hydrogen or hydroxyl ions in water. Taking methyl acetate, Wijs (1893) found a value of 1-2 x 10~7 at 25°.

Putting the four values together and converting them to the same temperature (25°), we find : — It is impossible to believe that values so near together could depend on accidental impurities. It should be remembered also that Arrhenius (1889, p. 103), on this hypoth. -i^. was enabled to predict the high temperature coefficient of its conductivity. On the other hand, Walden (1910) finds that water has no higher conductivity when dissolved in prussic acid, contrary to binary electrolytes of the ordinary kind. It seems, however, that there are anomalous conditions present, owing to chemical combination with the solvent.

Water, as Nernst points out, is capable of a second electrolytic dissociation, since But the separation of the second hydrogen ion from such a dibasic acid always takes place with great difficulty, so that the concentration of oxygen ions would probably be so small as to escape detection. There are a few more facts in connection with this question which require mention. Denham (1908) has shown that the hydrogen electrode can be used with good results in determining the degree of hydrolysis. The most interesting facts, for our purposes, obtained in this way are that ammonium chloride is only hydrolytically dissociated in water to a minute extent, namely, 0'018 per cent, for a O'Ol molar solution at 25°, while aniline hydrochloride 0'031 molar is 2'6 per cent, dissociated, whence ammonium is about seventy thousand times as strong a base as aniline.

The hydrolytic dissociation of Indicators is of importance as showing that their strength as acids or bases must not be too small ; otherwise the end point is inaccurate. The rule is that weak bases and weak acids are not to be used together ; that is, weak acid indicators are not to be used for titrating weak bases, nor weak bases for titrating weak acids. For more details see Nernst's book (1911, p. 535). The fact that hydrolysis can be reduced by the addition of excess of acid or base, respectively, enables precipitations to be avoided where the product of hydrolysis is insoluble. Thus acetic acid is added to mercuric acetate. Conversely, by reducing the H' ion concentration in ferric chloride solutions by the addition of sodium acetate, ferric hydroxide is precipitated. Or silicic acid may be precipitated from sodium silicate by addition of ammonium chloride. This kind of action is obviously of much importance in the reactions of analytical chemistry (Nernst, 1911, p. 548).

Finally, the circumstance that, when the weak base or acid of a hydrolyticallydissociated salt has a very small conductivity, it is found that addition of excess of this component beyond a certain degree causes no further change in the molar conductivity of the solute, as shown by Bredig (1894, 1, p. 214), enables the degree of hydrolysis to be determined by an independent method. Such a case is that of aniline salts. The phenomenon known as " catalysis " will come up for discussion in a later chapter. It will suffice here to state that there are substances which produce a great increase in the rate of reactions, although they themselves are not constituents of the final system in equilibrium and, as a rule, reappear finally in the same state as they were to begin with.

Hydrogen ions constitute one of the most powerful of these catalysts and, although they exist only in very small amount in water, their action must not be neglected. Hydroxyl ions are not catalysts, at all events in the saponification of esters, since they are used up in the reaction, thus : — The result of this reaction is to increase the hydrogen ions and to diminish the hydroxyl ions. There is, then, a double process, the details of which may be found in Nernst's book (1911, p. 567).

Most oxidation processes were supposed, up to recent times, to be simply explained by the direct union of oxygen with the substance to be oxidised ; but it has been shown conclusively, chiefly by the work of H. B. Dixon and H. B. Baker, that the presence of water is necessary. This fact will require further discussion in our chapter on oxidation, so that attention is directed to it here as another case in which water acts as a catalyst. It should be mentioned that Armstrong does not admit that it is water itself which acts in these cases, but the impurities contained in it, acting as conducting systems to bring the other components into reaction. A striking case, which seems to support this view, is that described by Brereton Baker (1902). It had been already shown by Dixon that water vapour is necessary for the explosion of a mixture of oxygen and hydrogen gases. Baker showed that if the gases are almost completely dried, a slow combination occurs on heating ; but although more than sufficient water is formed to bring about an explosion, none happens. The explanation, according to Armstrong, is that the water formed is too pure to allow the necessary conducting system between the reacting gases to be produced.

A large number of the reactions occurring in living organisms are those in which water is removed or added. The addition of water, hydrolysis, results in the splitting up of a complex molecule into smaller ones, and plays a large part in the phenomena of digestion, where certain agents, enzymes, are present whose function it is to hasten the process catalytically. As a simple instance, we might take glycyl-glycine : — By the entrance of a molecule of water at the arrow, the compound is split into two molecules of glycine : —

If two molecules of glycine be taken and a molecule of water removed, that is, H from the one, and OH from the other, synthesis of glycyl-glycine occurs. Consider, further, the equilibrium in a mixture of methyl acetate and water. Here, when water is added to methyl acetate in the proportion of one molecule to each molecule of the ester, part of the water hydrolyses part of the ester similarly to the previous case. But, when a certain fraction of the ester is hydrolysed, the process comes to an end, owing to the increase of the opposite synthetic reaction by mass action of the products of hydrolysis. Expressed in the usual way, we have in equilibrium : —

Suppose that we now increase the concentration of the water. It is plain that the only way K can remain constant is by diminution of Cester, which involves, at the same time, increase of the components of the denominator. Similarly, decrease of water means increase of ester, or synthesis. It is clear that, in this way, by alteration of the actual or effective concentration of water, the living cell has the possibility of changing the position of equilibrium in such reversible reactions, and thus causing the preponderance of hydrolysis or synthesis. It seems most probable that mechanisms of such a kind are active in the protoplasmic system, and that the taking up or giving off of water by colloidal substances is the chief one. In any case, we see the importance of the presence of water, not merely as a solvent to allow the reagents to come together, but also as an actual component of the chemical reactions themselves.

In pure water, the process of attainment of equilibrium is extraordinarily slow, so that it must be hastened by a catalyst. Hence the universal presence of enzymes in the organism. Although water, as such, is chemically so inert a substance, certain chemical individuals, such as sodium, enter into violent reaction with it. Here again, however, we are met with the possibility that the reaction is accelerated, or even rendered possible, onl}' by the presence ol some other substance, which acts as a catalyst. H. Brereton Baker (1910) and Baker and Parker (1913) have shown how greatly the rate of reaction between sodium amalgam and water is retarded by purification of the water.

The necessity of the presence of water for the manifestation of vital properties is sufficiently obvious from the former part of this chapter. An interesting question arises as to how far protoplasm can be deprived of water, while remaining capable of recovery to life, when again supplied with moisture. That drying in ordinary air is not necessarily fatal is shown by every-day experience with seeds, which can be kept a large number of years without losing their power of germination.

Shattock and Dudgeon (1912) have shown, moreover, that certain bacteria, even when they do not produce spores, can be exposed to a vacuum, produced by charcoal surrounded by liquid air, for a space of one hundred and sixteen days. One would suppose that all water would be removed from the organisms in this way. Mr Shattock informs me, that he has found, since the paper referred to was published, that after two years in the vacuum, Bacillus pyocyaneus was still capable of vigorous growth.

Apparently, under such conditions, all chemical processes cease, so that we must assume that the protoplasm remains in the state in which it was at the moment of desiccation and prepared to resume activity on the arrival of water. It is interesting to note that the bacillus in question lives longer in the dry vacuum than when merely air dried ; in this latter state it never survived longer than nine days, no doubt owing to chemical changes still continuing. Some kind of change can be brought about, even in the perfectly dry condition, since, if exposed to sunlight or ultra-violet radiation, it was found by Shattock and Dudgeon that bacteria were killed rapidly, even in the absolutely dry vacuum.

Naturally, the much more complex and sensitive organisation of the higher animals cannot be dried in this way. It is well known, however, that creatures as highly developed as Rotifers survive drying in air ; but this appears to be due to the production of a capsule which prevents complete loss of water. Davis (1873) saw a drop of fluid exude when he punctured the cyst of Philodina. It seems possible that desiccation at the eutectic temperature bj- Altmann's method, described in the first chapter of this book (page 17), might allow of recover}- of the cells of higher organisms. If so, a valuable means of investigation would be available: tissue. dehydrated in this way, can be cut into thin sections and the cells observed under tinmicroscope. The difficulty, as previously mentioned, comes in when it is required to add water again.

An important practical application of the facts described above, as to the necessity of the presence of water for protoplasmic activity, lies in the greater resistance of organisms to the action of heat the drier they are. This is, however, not invariably the case — Bacillus pyocyaneus is killed by exposure to 65° for an hour, wet or dry. The resistance is particularly noticeable in the case of spores of bacteria and other fungi ; as is well known, a higher temperature of sterilisation is required to kill them. This behaviour is also shown by enzymes, which resist a considerably higher temperature in the dry state than when in solution.

A fact worth recording here is that, as shown by Dreyer and Ainley Walker (1912), spores of bacteria suspended in glycerol or oil are not killed by exposure to a temperature of 119° C. for over half an hour. This fact is obviously of much practical importance, since sterilisation in non-watery liquids is frequently made use of. That organisms are under more or less risk of injury from drying is shown by the precaution taken by many of them to avoid the risk by surrounding them-

selves with a layer of substance comparatively impermeable to water, forming what are known as "cysts," as mentioned above in reference to Rotifers. It will also readily be understood that, in the dry state, protoplasm can withstand freezing temperatures better than in the normal active moist state. Seeds, although they are not absolutely devoid of water, can be exposed to the temperature of liquid air without injury. The need of water causes certain organisms to turn towards the place where it is to be found. This fact is very marked in the case of roots, leading to the phenomenon known by the above name. The side of roots turned away from the water grows more rapidly than that turned towards it, so that curvature results. The opposite behaviour is shown by the sporangia of Mucor, leading to bending away from the moist surface.

The subject of viscosity is, strictly speaking, not quite in place here, since it concerns other liquids in addition to water. But since, in physiological work, the liquids with which we have to deal are, almost entirely, solutions or suspensions in water, we may be allowed to take the subject at this stage, as a convenient one. As was pointed out by ^Newton, the particles of liquids are not free to move about without resistance due to their "adherence" to one another. This gives rise to friction, so that the viscosity, or internal friction, of a liquid is proportional to the velocity with which these particles are moving past one another and also to the extent of the rubbing surfaces.

The methods used for its determination consist either in measuring the resistance offered to the movement of a surface passing through the liquid, or in that of the resistance offered to the passage of the liquid through a narrow tube ; the latter method is a simple one and requires merely the determination of the time taken by a given amount of the liquid, under a given pressure, to run through the tube. The flow through tubes is not only the most important aspect of this property of liquids met with in ordinary life, but also in physiology, where the internal friction of the blood gives rise to what is often called the " peripheral resistance " of the vascular system. This it is, that, with a given rate and strength of heart beat, determines the arterial pressure.

The first point to be noted is, that when a liquid is being caused to flow through a tube by the pressure applied at the inlet end of the tube being greater than that at the outlet, the layer in immediate contact with the wall of the tube is at rest, while that in the middle has the greatest velocity ; each layer experiences friction at its contact with the neighbouring layer, so losing in velocity progressively until the outermost layer is reached, where the velocity disappears entirely. We see, then, that the friction is between the parts of the liquid itself and riot between the liquid and the wall of the tube.

Suppose, next, that the tube is a wide one and that the internal friction of the liquid is not great ; the thickness of the layer at the periphery in which the velocity is increasing from zero to its maximum rate will only be a narrow one The remainder of the column moves in all its parts with the same velocity, so that, in this part of the stream, there is no friction. Such tubes are the large arteries and veins. In a narrow tube, such as an arteriole, the layer whose constituent elements are in motion relatively to one another will reach to the axis of the tube, so that the whole of the liquid column is exposed to internal friction. We see, then, how, even supposing that the number of arterioles into which a large artery divides is sufficiently great to give a total cross-sectional area equal to that of the large artery, so that the rate of flow is no greater, the total mass of blood is causing fractional resistance, whereas in the large

artery merely a small fraction of it was doing so. The sectional arcu of the arterioles taken together may clearly be even greater than that of the artery, without affecting the nature of the result, although the effect will be less, on account of the less rate of flow. It is important to bear in mind that the peripheral resistance of the arterial system, resulting from the division into small arterioles, is due entirely to the internal friction of the blood, not to friction against the walls of the vessels ; except indirectly, in so far as it is this latter friction which determines the stationary condition of the blood film in contact with them.