THE ACTION OF ANTISEPTICS 169 



Thus it may be said that the most important antiseptics are the 

 salts of the heavy metals, certain acids, especially mineral acids, 

 certain oxidising and reducing agents, a great variety of sub- 

 stances belonging to the aromatic series, and volatile oils generally. 

 In comparing different bodies belonging to any one of these 

 groups the chemical composition or constitution is very important, 

 and if such comparisons are to be made, the solutions compared 

 must be equimolecular — in other words, the action of a 

 molecule of one body must be compared with the action of a 

 molecule of another body. This can be done by dissolving the 

 molecular weight in grammes in a litre of water. When this 

 is done, important facts emerge. Thus, generally speaking, 

 the compounds of a metal of high atomic weight are more 

 powerful antiseptics than those of one belonging to the same 

 series, but of a lower atomic weight. Among organic bodies, 

 again, substances with high molecular weight are more powerful 

 than those of low molecular weight — thus butyric alcohol is more 

 powerful than ethylic alcohol — and important differences among 

 the aromatic bodies are associated with their chemical constitu- 

 tion. Thus among the cresols the ortho- and para-bodies re- 

 semble each other in general chemical properties, and stand apart 

 from metacresol ; they also are similar in antiseptic action, and 

 are much stronger than the meta-body. The same may be 

 observed in other groups of ortho-, meta-, and para-bodies. 

 Again, such a property as acidity is important in the action of a 

 substance, and, generally speaking, the greater the avidity of an 

 acid to combine with an alkali, the more powerful an antiseptic 

 it is. With regard to oxidising agents and reducing agents, 

 probably the possession of such properties has been overrated as 

 increasing bactericidal potency. Thus in the case of such re- 

 ducers as sulphurous acid and formic acid, the effect is apparently 

 chiefly due to the fact that these substances are acids. Formic 

 acid is much more efficient than formate of sodium. In the case 

 of permanganate of potassium, which is usually taken as the 

 type of oxidising agents in this connection, it can be shown that 

 the greater amount of the oxidation which takes place when this 

 agent is brought into contact with bacteria occurs after the 

 organisms are killed. Apart from the chemical nature of anti- 

 septic agents, the physical factors concerned in their solution, 

 such as electrolytic dissociation and the number of free hydrogen 

 ions present, are important. The part played by such factors is 

 exemplified in the fact that a strong solution acting for a short 

 time will have the same effect as a weaker solution acting for a 

 longer time. From what has been said it will be realised that 



