STEEILISATION 91 



Finally, we have to consider various chemical substances that act as 

 disinfectants. AVe must consider the qualities that make a good dis- 

 infectant. Andrews has laid down the following : 



1. It should be germicidal within a reasonable time limit. 



2. It should not possess chemical properties which make the 



disinfected substances unfit for ordinary use. 



3. It should be soluble in water, or capable of giving rise to 



soluble products in contact with the material to be disinfected. 



4. It should not produce injurious effects on the human tissues. 



5. It should not be too costly in proportion to its germicidal value. 

 The following chemicals have been recommended : 



Mineral Acids. These include sulphuric-, nitric-, hydrochloric-, and 

 phosphoric-acids. There are very few bacteria that can live even in a 

 slightly acid medium. Thus, in the case of a weak acid like phosphoric 

 acid, one drop of a 1'75 per cent, solution added to 5 c.c. of broth 

 inoculated with the spores of Sarcina ureae, is sufficient to make the 

 medium quite unsuitable for the development of this germ. The 

 application of mineral acids is limited because of their disastrous effect 

 on the skin, and their corroding action on metallic substances. Also, 

 as is well known, acids destroy all cloth substances. They are useful 

 when we wish to sterilise small quantities of liquid containing patho- 

 genic bacteria before throwing the liquid away. 



Alkalies, e.g. potash (KOH), sodium hydrate (NaOH), etc. Most 

 bacteria can thrive in very slightly alkaline media, but growth is 

 prevented when the reaction becomes decidedly alkaline. The effect 

 of a 5 per cent, soda solution on the spores of Sarcina ureae is shown by 

 the following experiment : Three drops of this solution were added to 

 •5 c.c. of broth inoculated with this organism. This was sufficient 

 to prevent the normal formation of spores, though at first growth 

 without spore-formation was not prevented. After five days the con- 

 ditions were so unfavourable that all the individuals in the culture had 

 succumbed. We may then say that all strong alkalies are disinfectants, 

 but that as they prejudicially affect the skin and clothes, like acids, 

 they can only be advantageously used to sterilise infected substances 

 previous to throwing them away. 



Carbolic Acid. Probably this is the best known disinfectant on 

 account of its effectiveness, its cheapness, and absence, in the diluted 

 condition, of injurious action on the clothes and skin. A strength of 

 1 in 40 has been found sufficient to destroj'' Streptococcus pyogenes, 

 S. erysipelatis, and Staphylococcus pyogenes aureus, whilst tubercular 

 sputum has been sterilised by being mixed with a 1 in 20 solution, 



