618 PRINCIPLES OF GENERAL PHYSIOLOGY 



with one degree of freedom only. But this is not in agreement with experimental 

 results, which show that the system is bivariant ; we can vary both temperature 

 and oxygen pressure, and yet obtain equilibrium. We must either assume that, 

 instead of three phases we have only two, or that there are three components, 

 and it is not easy to see how this happens. Another alternative is that the phase 

 rule does not apply to the case of micro-heterogeneous systems. There is reason 

 to believe that curvature of surface plays a large part in the properties of the 

 colloidal state (see page 51 above). This fact may perhaps bring reactions in which 

 ultra-microscopic particles are concerned more into approximation to those between 

 molecules. There may thus be a region in which transitional states between 

 simple surface adsorption and true chemical combination are to be met with. 

 The question requires further investigation. 



There are two important principles to be remembered in connection with the phase rule. 

 The one, van't HoflTs " principle of mobile equilibrium," has been discussed previously 

 (pages 44-45). The other, that of Le Chatelier, is related to it and may be stated thus : If 

 a system in equilibrium is subjected to a constraint by which the equilibrium is shifted, a 

 reaction takes place which opposes the constraint, that is, one by which its effect tends to 

 be annulled. Take water, ice, and steam at 0, addition of heat causes melting of ice by 

 which the heat becomes "latent," and there is no rise of temperature until the whole of 

 the solid phase is melted. 



Sodium Bicarbonate. A solution of sodium bicarbonate in water in contact 

 with various pressures of carbon dioxide appears at first sight to come nearer to 

 the kind of system we want, since, even after allowing for the increased solubility 

 of carbon dioxide with pressure, we find that more is taken up as the pressure 

 increases and in certain proportion to the pressure, although the amount is not 

 great and the range is a short one. A little closer examination, however, shows 

 that the system is in no way analogous to that of haemoglobin. In sodium 

 bicarbonate we have a salt which is electrolytically dissociated in water, so that 

 there is an equilibrium between the several ions and the undissociated salt. When 

 the pressure of carbon dioxide outside is raised, more HCO 3 ' ions are formed in 

 the solution. The result of this is that the dissociation is put back and more 

 sodium exists in the state of combination as bicarbonate, as is seen by the 

 dissociation equation : 



(Na-) (HCO 3 ') = K(NaHCO 3 ) (see page 185). 



In the system, therefore, there is more additional CO 2 than is to be accounted 

 for merely by the increase of dissolved gas, but the increase is due to the fact that 

 the salt is electrolytically dissociated. Oxyhaemoglobin does not dissociate in this 

 way into oxygen ions, and haemoglobin ions, and, in fact, like other proteins and 

 amino-acids, it is an amphoteric substance, and to all intents and purposes a non- 

 conductor. We find on p. 22 of Barcroft's book (1914) that a solution of 

 haemoglobin, which had only been dialysed for three days, had an electrical 

 conductivity equal to that of 0'004 molar sodium chloride only ; further dialysis 

 would have reduced it still more. 



Reducible Dyestuffs. There are a number of dyestuffs which are capable of 

 existing in two forms, an oxidised and a reduced form. In the presence of oxygen, 

 in many cases, the reduced form (leuco-base) is oxidised. Prof. W. A. Osborne 

 informs me that he hoped to find amongst these a case like haemoglobin, but was 

 unable. All of them behaved like calcium carbonate ; that is, under a given 

 oxygen pressure, the dye was either completely oxidised or completely reduced, 

 according to the pressure. Again a case of all or nothing. 



According to the work of Alsberg and Clark (1914), haemocyanin is similar to 

 these dyes. It is blue in the arteries and colourless in the veins, that is, the 

 reduced form takes up oxygen and becomes blue. But this oxygen is not given 

 off to a vacuum ; the blood merely gives up the gas dissolved in the water. It is 

 suggested that the copper contained in the pigment may act as a catalyst, as we 

 have seen above (page 585), the oxygen being thus more readily given off to an 

 acceptor, such as may be present in the tissues. If so, h;?mocyanin would be 

 analogous to a peroxide-peroxidase system. 



Adsorption. It may occur to the reader that there is one class of cases of which 



