22 THE EQUILIBRIUM OF COLLOID AND 



serum appears to be acid when tested by phenol-phthalein, and 

 appears to be alkaline when tested by methyl orange, &c. Not, 

 as is too often stated, because it is at the same time acid and 

 alkaline, for that is absurd, but that its actual position in reaction 

 lies very nearly at the neutral point, and just short of that slight 

 degree of alkalinity which shows the alkaline colour to phenol- 

 phthalein on the one hand, and just short of that degree of acidity 

 which gives the acid colour to methyl orange. Now these two 

 points lie very close together, for if instead of the serum we take 

 distilled water and add the two indicators phenol-phthalein and 

 methyl orange in traces to it, then a single drop of dilute alkali 

 will develop the alkaline colour of the phenol-phthalein, and on 

 the other hand a single drop of dilute acid will show the acid colour 

 of the methyl orange. 



In the case of the serum, however, the result is quite different, 

 for very considerable amounts of alkali must be added before it 

 turns alkaline to phenol-phthalein, and proceeding in the opposite 

 direction still larger amounts of acid must be added before acidity 

 to methyl orange is realised. The reason for this is that the pro- 

 teins, which can figure either as acid or base according to whether 

 there is excess of alkali or acid respectively in the solution, must first 

 be satisfied before the indicators are affected ; and as the amount 

 of protein is large, so the amount of acid or alkali required before 

 it is neutralised and the acidity or alkalinity can commence rapidly 

 to run up and affect the coloured indicator, is very considerable. 



This is a factor of great importance to the life of the cells, which 

 cannot bear any appreciable degree of either acidity or alkalinity, 

 and are protected from such variations by the very delicate regu- 

 lation of the reaction by the amphoteric proteins. 



The regulating action of the proteins upon the reaction of the 

 serum has been mentioned here, however, because it gives a strong 

 indication that the proteins are in union with the inorganic salts. 

 If a clear sample of serum be titrated with methyl orange or " di- 

 methyl " as an indicator, an alkalinity equivalent to the very high 

 figure of 0-17 to O18 normal is obtained. This alkalinity is chiefly 

 due to proteins', for if the salts of the serum be separated off by 

 dialysis or incubation and titrated to the same indicator, the 

 alkalinity now amounts to only O03 to O04 normal. Subtracting 

 these amounts due to inorganic constituents from the higher figure, 

 we obtain the result that the combining power of the serum pro- 



