DISINFECTANTS 39 



C, is a process frequently adopted for the preservation 

 of foods for a limited time. It is largely employed in 

 the dairy industry, and is effective in destroying the 

 sporeless forms of bacteria without materially altering 

 the flavour or composition of the liquids. For its 

 application to milk and cream, see pp. 295-6. 



8. Disinfectants, Antiseptics, and Preservatives. 

 The vitality and work of bacteria can be checked or 

 destroyed by various chemical substances. Surgical 

 instruments and premises infected with disease germs 

 are generally sterilized by the application of these 

 compounds. Chemical substances which completely 

 annihilate bacteria are termed disinfectants ', while 

 those which merely retard the growth of these 

 organisms are known as antiseptics. No hard line of 

 separation, however, can be drawn between these two 

 classes, for the effect of any chemical compound depends 

 upon the strength of its solution and the time that it is 

 allowed to act ; weak solutions of disinfectants may have 

 an antiseptic action only. Moreover, what would be 

 sufficient to destroy or impede the growth of one species 

 of bacterium will not be equally effective upon another 

 kind. 



The following are among the most widely used dis- 

 infecting and antiseptic agents : 



Mercuric chloride (HgCl 2 ), in a . I to . 2 per cent, solution, 

 is one of the most certain of disinfectants, but its 

 excessively poisonous quality precludes its general use 

 except in the laboratory under proper control. It is 

 extensively used in surgical practice. 



Sulphur dioxide gas (SO 2 ), or solutions of it in 

 water (sulphurous acid), are effective destroyers of 

 bacteria and fungi. The vapour of burning sulphur, 



