THE ACTION OF ANTISEPTICS 143 



to the bacteria is swamped in an excess of culture fluid, can safely be 

 followed, especially when a series of antiseptics is being compared. 



Much attention has been paid to the standardisation of antiseptics, 

 and a watery solution of carbolic acid is now generally taken as the 

 standard with which other antiseptics are compared. Rideal and 

 Walker point out that 110 parts by weight of B. P. carbolic acid equal 

 100 parts by weight of phenol, and they recommend the following method 

 of standardising. To 5 c.c. of a particular dilution of the di.-infectant 

 add 5 drops of a 24-hour-old bouillon culture of the organism (usually 

 b. typhosus) which has been incubated at 87 C. Shake the mixture and 

 make subcultures every 2^ minutes to 15 minutes. Perform a parallel 

 series of experiments with carbolic acid and express the comparative 

 result iu multiples of the carbolic acid doing the same work. 



The Action of Antiseptics. In inquiries into the actions of 

 antiseptic^ attention to a great variety of factors is necessary, 

 especially when the object is not to compare different antiseptics 

 with one another, but when the absolute value of any body is 

 being investigated. Thus the medium in which the bacteria to 

 be killed are situated, is important ; the more albuminous the 

 surroundings are, the greater degree of concentration is required. 

 Again, the higher the temperature at which the action is to take 

 place, the more dilute may the antiseptic be, or the shorter the 

 exposure necessary for a given effect to take place. The most 

 important factor, however, to be considered is the chemical 

 nature of the substances employed. 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 re- 

 ducing 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 

 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 say a litre of water (see p. 33). 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 e thy lie alcohol and important differences among 



