28 INTRODUCTION TO GENERAL PHYSIOLOGY 



region. Moreover, unlike some other indicators, it is not particularly 

 sensitive to the presence of salts or proteins in the solution. Thus, 

 the H-ion concentration at which a change of tint occurs is 

 practically the same in their presence or absence, so long as they 

 are not in great excess. 



That it is the concentration of hydrogen-ion that an indicator 

 really gives information about is well seen by taking a strong 

 solution of hydrochloric acid and diluting it with water. The dye 

 known as " crystal violet " will be found to show a series of definite 

 changes, although nothing has been done except to decrease the 

 concentration. That it is the hydrogen-ion, and not the anion, 

 may be seen by taking a different acid, say sulphuric, when we find 

 the same series of changes (E., p. 175). 



The student may notice that this use of indicators differs some- 

 what from the usual one of determining the amount of total acid 

 present by " titrating " it with a standard solution of alkali, or 

 vice versa. In such cases, as is seen in practice, the degree of 

 dissociation of the acid does not play any part. Molar solutions 

 of hydrochloric and of acetic acids require the same amount of 

 caustic soda to neutralise them to an indicator. How is this to 

 be explained if the concentration of H ions is so much greater 

 in the former case ? We have only to remember that, as each 

 successive portion of H ions is combined with OH ions from the 

 alkali to make water, the remaining part of the acid, becoming less 

 and less concentrated, continues to become more and more 

 dissociated, until the whole of it, whatever the original degree 

 of dissociation, has passed through the ionised state, and the H ions 

 have been neutralised by OH ions. 



The Electrical Resistance of Living Cells 



Since the electrical current can only pass through solutions of 

 electrolytes by virtue of its carriage in charges on moving ions, 

 it is obvious that the conducting capacity of a solution depends 

 on the width of the channel between the electrodes, as well as on 

 its length. If, therefore, part of this channel is filled up with some 

 non-conductor, such as grains of sand, there must be an obstruction 

 to the passage of a current. Further, if the cell membrane is im- 

 permeable to the ions of a solution in which the cells are immersed, 

 these cells must behave simply as inert bodies, blocking the passage 

 of a current in fact, as if they were grains of sand. This is found 

 by experiment to be the case, and has been used to determine the 

 number of blood corpuscles in a given volume of blood (E., p. i/'5). 

 If the cells are killed, the membrane becomes permeable, and the 

 conductivity rises, because the cells now admit of the ions of the 



