244 AIDS TO BACTERIOLOGY 



were inconstant. In the Porter-Clark process, where the 

 precipitated calcium carbonate is removed by filtration 

 through canvas, considerable reduction in bacterial con- 

 tent results (Hewlett and Nankivell). 



Houston's ' excess lime ' method involves the addition 

 of lime till the water is decidedly alkaline. After a period 

 of five to twenty-four hours, the time required to kill 

 colon bacilli, sufficient stored water is added to deal with 

 the excess of lime. In his Tenth Research Report 

 Houston mentions that, with small quantities of lime 

 above the neutralisation point, colon bacilli lived over 

 two days, whilst with 6 -3 parts of lime in excess they 

 lived for less than one day. 



CHAPTER XXII 



DISINFECTION AND DISINFECTANTS 

 Fire. 



DESTRUCTION by fire is the most efficient and simplest 

 form of disinfection, and should be employed where 

 articles are of little value, or to remove infection from 

 surfaces which will not be appreciably damaged thereby. 

 Forbush and Fernald's apparatus consists of a portable 

 tank and pump, from which paraffin gas oil is driven 

 through a hose (such as is used for the delivery of oil), to 

 which is attached a pole, consisting of an iron pipe 12 feet 

 long, which is protected by a covering of wood, and 

 to the end of which is attached a cyclone nozzle. The 

 fine spray from the nozzle is ignited, and the resulting 

 torch-fire passed over the surfaces to be disinfected. 



Heat. 



Most articles sent for disinfection are of an organic 

 nature, to which injury is inevitable at temperatures not 

 much above those necessary for disinfection. Death of 

 the bacteria is determined by the coagulation of the 

 protein. With unlimited moisture protein free from salts 

 can be coagulated as low as 50 C.; in the absence of 

 moisture it can stand exposure to 170 C. 



