THE TASTE OF ACIDS 



459 



below about n/200 is only 6 per cent dissociated at that con- 

 centration, whereas hydrochloric acid, fully dissociated (99 per 

 cent), is tasted down to about n;900. The actual hydrogen 

 ion concentrations in these solutions limiting tastability are: 

 HCl (n 900), C„ = 0.00119 N; acetic (n '200), C, = 0.00035 N. 

 Acetic acid is more stimulating than would be calculated from 

 its dissociation. 



Exactly similar conditions are fountl in studying the pene- 

 tration of cells by acids, as will be clear from an inspection of 

 the two sets of curves in figure 2. In table 1 acid solutions giving 



The ionization of hydrochloric, acetic, and butyric acids, in solutions which pene- 

 trate cells within equal titnes. For sources of data see corresponding reference 

 numbers in figure 2. 



the same penetration time appear in the same horizontal row; it 

 is seen that the penetration of cells by acetic acid is much more 

 efficient than if its dissociation were the deciding influence. 



It follows that the stimulus produced by acetic acid is due, 

 in the first place, to its union with the cell surface; this pene- 

 tration of the plasma membrane is more efficient than if dissocia- 

 tion strength were the determining factor. The indication is 

 that this acid dissolves in a fatty substance located at the cell 

 surface (for further evidence, see Crozier, '16). The stimulus 

 due to hydrochloric acid also depends upon its penetration of 

 the cell surface, but there is some evidence that it involves 

 proteins. 



It is not clear to what extent stimulation by acetic and hy- 

 drochloric acids may be interpreted in terms of the effects which 



