310 THE EQUILIBRIUM OF COLLOID AND 



and adsorptions which lie close together, it is found that the 

 characteristic differences become diminished in degree, there are 

 all possible gradations, and many instances in which it is impossible 

 definitely to say whether the union which takes place ought to be 

 described as an adsorption or a chemical combination. 



If the various criteria of a physical or chemical nature which 

 are usually taken as decisive of whether any union is a chemical 

 combination or an adsorption be examined, it is found that they 

 one after another break down. 



To take an example oHnterest to the biologist, it was taught 

 for many years in all the physiological textbooks that haemo- 

 globin formed with oxygen an easily dissociable compound. The 

 chemical combination was said to be complete at a certain pressure 

 of oxygen, and as the oxygen pressure fell this compound, called 

 oxyhaemoglobin, dissociated off into oxygen and haemoglobin, or 

 " reduced haemoglobin," the dissociation occurring over a definite 

 range of pressures. So certainly established were the facts regard- 

 ing this that the proofs of the formation of the compound formed 

 a stock question of the examination room. One of the strongest 

 proofs of this formation of the compound oxyhaemoglobin was 

 supposed to be that the amount of oxygen absorbed by haemoglobin 

 was not directly proportional to the partial pressure of the oxygen, 

 absorption occurring in relatively greater amount at the lower 

 pressures and falling off rapidly to nearly a zero increment as the 

 dissociation range was passed. So that when oxygen pressures 

 were graphically plotted as abscissae and amounts absorbed as 

 ordinates, instead of a straight line, as, say, in the case of absorption 

 of oxygen (or other inert gas) by water, a curve was obtained. 



But recent research has shown that in many cases where the 

 phenomena ought to be classed under the head of adsorption, the 

 plotted curve of pressure (or concentration) and of amount ab- 

 sorbed is not a straight line but a curve, and hence the new criterion 

 is not a simple linear relationship between concentration and 

 amount absorbed, but that the plotted curve shall show kinks or 

 breaks upon it that is to say, regions at which there is a sudden 

 change in the equation of the curve. Now the oxygen-haemoglobin 

 curve is a smooth curve, and for this reason it has recently been 

 maintained by Wolfgang Ostwald that in the case of oxygen and 

 haemoglobin the phenomenon is one of adsorption. Yet in the case 

 of oxygen and haemoglobin there exists at the point where^absorp- 



