ELECTROLYTES AND THEIR ACTION 



2OI 



the absorption coefficient of CO 2 is unity, that is, about 17. Then the stages 

 will be about as given in the following table : 



Until nearly 250 g. of hydrochloric acid have been added, neither the acidity 

 nor the alkalinity is greater than twice that of a perfectly neutral solution. 

 The cause of this constancy is simple enough. At the beginning the free CO 2 

 of the solution is in equilibrium with that of the gas phase. Accordingly when 

 hydrochloric acid is added and reacts to form sodium chloride and more .CO.,, the 

 whole of the latter escapes to the gas phase and the total amount of acid is 

 what it was before, viz , saturation with CO 2 at a partial pressure of 1 g. per 

 litre of air, since all the hydrochloric acid has combined with the bicarbonate. 

 Thus the concentration of the alkaline salt (bicarbonate) is diminished, but 

 there is no increase of free acid. Not until all the bicarbonate is decomposed 

 does the hydrochloric acid begin to show its effect, and then the addition of 

 2 g. causes nearly as much rise in acidity as the previous 318 g. had done, or 

 about 200 times the rise caused by 100 times the amount at the first stage 

 of the experiment. 



A remarkable fact was noticed by Henderson (1908, p. 176) in comparing the relative 

 amounts of alkali necessary to produce a given change in the H' ion concentration, as shown by 

 indicators, in the cases of various weak acids. With the single exception of hydrogen 

 sulphide, it was found that NaH z PO 4 and ff 2 CO 3 required the largest quantities. Acids both 

 weaker and stronger than these required very much less, there being a large step between the 

 three mentioned and the next in the series. 



The "Fitness" of Carbon Dioxide. It will probably not have escaped the 

 reader that, as is insisted upon by L. J. Henderson (1913), it is a remarkable fact 

 that it should be carbon dioxide, the universal product of oxidation in the living 

 organism, that is the most efficient regulator of neutrality. Of course it is clear 

 that organisms would not have been able to develop to their present degree of 

 perfection without some mechanism of this kind, and that it is in adaptation to a 

 system in which carbon compounds play the chief part that their mechanisms have 

 been evolved. None the less it is calculated to excite a certain amount of wonder 

 that the element carbon, which is, as pointed out above (page 41), so peculiarly 

 adapted for the formation of a great variety of complex compounds, should also 

 include amongst these an acid with the properties which carbon dioxide alone, 

 with the exception of hydrogen sulphide, possesses. Especially is this so when we 

 remember that there is no reason to suppose that this property is necessarily 

 connected with the other properties of carbon. In the next chapter we shall see 

 that similar remarks apply with even more force to the case of water. 



In this connection we may call to mind what Parker (1913, 1) points out, namely, that 

 man}' apparent adaptations are not really such. That a person who faints falls with muscles 

 limp is appropriate for recovery, and it is also the safest way to fall, but these conditions are 

 the direct consequence of the faint, and that they are advantageous is purely incidental ; they 

 might, in fact, have been the opposite, but they would happen, notwithstanding. Parker 

 holds that the majority of animal reactions are, probably, neither of advantage nor dis- 

 advantage, in any notable degree, to the life of the individual, but dependent on the con- 



