616 PRINCIPLES OF GENERAL PHYSIOLOGY 



continues to do so until the whole of the carbonate is decomposed and pure 

 calcium oxide remains (see Findlay's book, 1904, p. 79). With oxyhnemoglobin, 

 on the contrary, reducing the oxygen pressure does not lead to total reduction, 

 but to a different state of equilibrium in which there is a smaller amount of 

 oxygen " combined " with the hemoglobin. If, again, we start with calcium 

 oxide at 547, and expose it to carbon dioxide at a pressure of 2'7 cm. of mercury, 

 the ichole is converted into carbonate ; if the pressure of carbon dioxide is less 

 than this, no change takes place at all. 



If the system is a closed one, so that there is only a limited amount of carbon 

 dioxide present, and at a pressure of 3 cm. of mercury, then a certain quantity 

 of the gas combines with a part of the calcium oxide until the pressure is reduced 

 to 2 '7 cm. of mercury : after that, nothing further happens. But this has nothing 

 to do with the haemoglobin system, since oxyhemoglobin may be in equilibrium 

 with an unlimited atmosphere of oxygen at any pressure, and remain at the 

 same percentage saturation indefinitely. 



It is perhaps useful to state the case also in terms of mass action. A detailed 

 account will be found on p. 55 of Cohen's book (1901) from which I take the 

 following condensed statement. As in all heterogeneous systems, it is not a 

 simple matter, at first sight, to understand what are to be regarded as the active 

 masses of the constituents. That of carbon dioxide is no doubt given by its 

 pressure. As to that of the solids, calcium carbonate and calcium oxide, the 

 consideration of water and of naphthalene will assist. Water, in a closed space 

 and at a given temperature, is in equilibrium with a certain definite pressure of 

 its vapour. Naphthalene, although a solid, behaves similarly, but its vapour 

 pressure is very small and difficult to measure. We may, therefore, assume that 

 calcium carbonate and calcium oxide are also in equilibrium with a definite 

 pressure of their vapours at a particular temperature. This pressure is sometimes 

 called the " sublimation tension." 



Now, just as the tension of water vapour is independent of the mass of the 

 water, so are the sublimation tensions of our calcium carbonate and calcium oxide 

 independent of their total masses. Since the chemical reaction takes place between 

 molecules, it must be in the vapour phase surrounding the solids. At a given 

 temperature, the concentrations of the vapours of calcium carbonate and calcium 

 oxide are constant, being proportional to the sublimation tensions. In general, 

 the active mass of a solid at a given temperature is therefore constant. Next, bv 

 the law of mass action, we have, in equilibrium, at a given temperature : 



= K 2 (CaO)(CO 2 ) 



where the concentrations are taken as being equal to the vapour pressures. Now 

 (CaCO 3 ) and (CaO) are constant, hence also Kj(CaCO 3 ) and K. 2 (CaO) are also 

 constant; call the former K 3 and the latter K 4 and we have : 



K 3 = K 4 (CO.,) = K 4 x pressure of CO. 2 . 



Thus, when calcium carbonate dissociates into calcium oxide and carbon dioxide, 

 at a given temperature, the pressure of carbon dioxide has a constant value, which 

 is independent of the relative proportion of the two solids. This is called the 

 dissociation tension of calcium carbonate at the temperature in question, and we 

 have seen above what happens when the tension of carbon dioxide is varied at 

 a given temperature. 



We see then that this system does not help us. It is sometimes said that it 

 is not analogous because there are changes of phase in it ; but there are also in 

 the case of oxyhsemoglobin solutions. This substance is in the colloidal state ; its 

 particles are sufficiently large not to pass through parchment paper ; it therefore 

 possesses surface, and is a separate phase. This fact is pointed out by Mines 

 (see Barcroft's book, 1914, p. 51). 



The Phase Rule. As reference has been made to the application of this rule 

 to oxyhaemoglobin, a few words are advisable to explain its general meaning. We 

 have already seen (page 48) that, in a heterogeneous system, each component which 

 does not mix with the others fs called a phase, and that there is a boundary surface 



