142 BACTERIA IN AIR, SOIL, AND WATER. 



important factor, however, to be considered is the chemical 

 nature of the substances, em ployed. Though nearly every sub- 

 stance which is not a food to the animal or vegetable body is 

 more or less harmful to bacterial life, yet certain bodies have a 

 more marked action than others. 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 substances belonging to 

 the aromatic series, and volatile oils generally. In comparing 

 different bodies belonging to any one of these groups the chemi- 

 cal 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 say a litre of water (see p. 36). When this is done 

 important facts emerge. Thus, generally speaking, the com- 

 pounds 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 sub- 

 stances 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 consti- 

 tution. Thus among the cresols the ortho- and para- bodies 

 resemble 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 the 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 

 reducers as sulphurous acid and formic acid, the effect is appar- 

 ently 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 



