126 



DISINFECTION 



can be neutralized by a salt of calcium. Students of physiology will recall the 

 similar observations made by Ringer on heart muscle in 1880. Shearer (1919) 

 found that living bacteria offered a considerable resistance to the passage of an 

 electric current, depending apparently on the relative impermeability of the cell 

 membrane. Using, therefore, electrical conductivity as his criterion of viability, 

 he obtained evidence suggesting that a 0-85 per cent, solution of NaCl was toxic 

 to the meningococcus, but that this toxic action could be neutralized by the addi- 

 tion of a trace of CaClj or other bivalent salt. On the other hand, it appeared 

 doubtful whether the toxic action of a bivalent could be neutralized by the addition 

 of a monovalent salt. 



Similar results have been obtained by other workers, v. Eisler (1909) found 

 that the inhibitory action of LiCl on B. subtilis could be counteracted by the 

 addition of a divalent, but not of a monovalent salt. Thus, N/10 LiCl was 

 counteracted by N/20 CaClj, by N/200 BaCla, or by N/200 MgS04. Further, 

 he showed that the inhibitory effect of a divalent salt could be counteracted by 

 either a mono- or a divalent salt. Thus, N/750 MnSOi was counteracted by 

 N/200 Ca(N03)2 and by N/100 KCl. This latter conclusion differs from Shearer's. 



It must not be thought that the mere addition of a divalent to a monovalent 

 salt will render the solution favourable ; the two salts must be present in definite 

 proportions. If not, instead of being harmless to the organism, the solution may 

 be actively toxic. Thus Winslow and Falk (1923) found that 0-14:5 M solution 

 of CaClj mixed with a solution of NaCl of two or three times this strength was 

 highly toxic to Bact. coli. As the proportion of NaCl was increased to four times 

 the strength of the CaClj solution, the toxicity of the solution diminished very 

 markedly. That is, a solution of 0-145 M CaCla + 0-290 M NaCl was toxic ; a 

 solution of 0-145 M CaClg + 0-680 M NaCl was non-toxic. A further increase of 

 NaCl rendered the solution again toxic. 



This antagonistic effect of salts brings us to the conception of a balanced 

 solution. A balanced solution is one in which the proportion of the different 

 salts is so ordered that their individually toxic effects are neutralized. In such 

 a solution, bacteria are able to survive very much longer than in a solution of 

 any one of the constituent salts. Ringer's solution is of this type, and has the 

 following composition : 



Winslow and Dolloff (1928) have, however, drawn attention to a possible fallacy in the 

 interpretation of the antagonistic effect of salts. According to them, aU cations appear 

 to stimulate growth in a certain low concentration and to inhibit it in a certain higher 

 concentration. So far as viability is concerned, therefore, they would postulate an optimimi 

 ionic concentration for each organism, depending probably upon an alteration in the 

 permeability of the ceU waU. They would explain the apparently antagonistic effects of 

 monovalent and divalent salts as being due not to a qualitative antagonism between the 

 two cations, but to the production in the suspension of a more favourable ionic concentra- 

 tion for the survival of the bacteria. In support of this they quote experiments in which 

 the toxic action of a given salt in dilute solution has been annulled by increasing the con- 



