CHEMICAL DISINFECTANTS 627 



8. Broth cultivations and agar plates both to be used in deter- 

 mining the death of the bacteria, and the period of observation of 

 these cultures to be extended, when necessary, to seven or fourteen 

 days. The experiments to be repeated at least once. 



9. Thermal death-point to be first roughly determined to within 

 5 C. 



10. Thermal death -point to be finally determined to within 

 1 C., and to be defined as that temperature which causes the 

 death of all micro-organisms exposed to it, within the ten minutes 

 in these standard conditions. 



LIGHT is not used directly for disinfection, but indirectly 

 in nature and in our homes may not be an unimportant 

 factor. It has previously been referred to at p. 23. Sun- 

 light, and artificial light rich in violet and ultra-violet radia- 

 tions, such as that emitted by a quartz mercury vapour 

 lamp, are efficient germicides. The latter has been 

 tested by Barnard and the writer with excellent results, 

 but, unfortunately, the germicidal rays have practically no 

 power of penetration and are stopped even by thin glass. 



DESICCATION, although one of Nature's methods of 

 disinfection, is not made use of to any extent by man 

 except as an inhibitory agent for the preservation of many 

 articles of food. Shattock and Dudgeon found that many 

 bacteria, e.g. B. coli. and B. typhosus, rapidly succumb to 

 complete desiccation, but B. pyocyaneus maintained its 

 vitality for two years under these conditions. 



FILTRATION is a method of disinfection by exclusion, 

 and in the form of sand filtration and filtration through 

 porous porcelain, as in the Berkefeld and Pasteur-Chamber' 

 land filters, is made use of for the sterilisation of water 

 and other fluids. 



CHEMICAL DISINFECTANTS. A large number of chemical 

 substances variously known as germicides, antiseptics, 

 disinfectants, deodorants, etc., have the power of inter- 

 fering with, or masking the results of, the vital activities 

 of micro-organisms. Germicides are substances which 



